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You need to be specific about what you mean by "race". The word gets commonly used in several distinct ways:


Groups of common descent and broad cultural identity. The more specific term for this would be "ethnic groups" (roughly; ethnic groups don't always have common descent, but they usually do). These, of course, exist in the sense that ancestry exists and groups of humans often self-identify with it.


Race in the anthropological sense, which is a categorization of people used by a culture, usually based on the views of the people who control that culture. These categories are usually based on appearance and shift over time. For example, olive-skinned Europeans were once not seen as proper 'whites' in American culture, and similarly, very-distantly-related people from Africa and from Melanesia the South Pacific might both be classified as "black" in American culture. Race in this sense "exists" in the sense that cultures do have such categories.


Race in the appearance sense, i.e., people can be categorized (not necessarily usefully) by their physical features. The features, of course, exists, which seems to be what you're arguing.


I think what you're arguing is, essentially, that only the third one of these "exists". But just because a thing comes from a culture and is dependent on that culture's norms doesn't mean it isn't important. Money is a cultural construction, but we care a great deal about that! So the second sense here "exists" in a way that will certainly affect your experiences in different cultures. And of course, common ethnic descent and identity exist as well, both in a literal biological sense in terms of descent and in an important cultural sense in terms of identity.

The only thing that differs is skin color. And skin color is not race. Skin color is your body adapting the area you live in. Should you leave and go to another country, over time, your descendants will have a change in skin color,

This isn't true, unless you're talking about timescales long enough for populations to evolve significantly. (And in modern times, such evolution mostly isn't happening along lines that make your local UV exposure matter anyway.) For the most part, Lamarckism simply isn't true.
You're sort of correct in that "race" isn't a well constructed categorization. It's neither consistent across societies nor does it actually break down humanity into discrete little boxes.
That said, it certainly exists and skin color certainly determines one's race in certain societies. It was socially constructed. We don't so much as choose the categories as the categories were chosen centuries ago and we're stuck with them until we're able to eliminate disparities along racial lines (read, likely never). 
E.g. in America at least white people have white skin. Black people have black skin. We call the racial category white people belong to "white" regardless of their ethnic origins or culture. We call the racial category black people belong to "black" regardless of their ethnic origins or culture.
Is it useful to do so? Only insofar as race has been and still is a source of disparity, disenfranchisement, and tool of oppression. Resolve those, dissolve race. >

Eh I guess. Imo "race" is only a word used by racists. The rest of us use words like ethnicity to describe a person's ethnic background. >

"Race" as a term gets used in academics, but it isn't equivalent to ethnicity.
Ethnicity is self-identified. Members of an ethnic group consider themselves and others of that ethnic group members of it, usually due to shared culture, ancestry, language, religion, or some combination thereof.
Race is a category applied by a culture to categorize people in general (not just themselves), and which might or might not actually line up with anyone's identity or ancestry. For example, a Japanese person and a Chinese person are both "Asian" in American racial categorization, but they're different ethnic groups that have, at times, really hated one another. >


Therefore, race, at least in humans, doesn't exist.

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but in humans, the concept of race is meaningless.  

Something being meaningless is not the same as not existing. And certain humans have put emphasis on race, so how could it be meaningless? >

Anthropologist can easily determine the racial origins of skeletal remains.. there are certain diseases which affect more races than others.. etc
this seems more like a post declaring how you wish it were and not how it actually is. >

I am of the Human Race, Homo Sapien.
Tell me, which Race is a black man of? >

This is an extremely outdated racial classification from like the late 1800s. Modern usages of those terms are pretty much limited to white supremacists, not actual scientists. >

In what way is the classification “extremely” outdated? 
We use it today in forensics and anthropology for a number of different reasons.. macro scale variations exist, groups of different peoples exist throughout the world - so I am not sure that I follow what you are saying.. >


We use it today in forensics and anthropology

Uh, citation extremely fucking needed.

macro scale variations exist, groups of different peoples exist throughout the world

They do, but those variations are mostly along continuous-ish spectrums, not neat sharp divisions into precisely three groups based on social-darwinists from 1895. >

The terms and categories you're using are nowhere in that link. Because, again, they're old school white supremacist categories from a century-plus ago. >

European would be caucasoid, sub Saharan is negroid and American Indian is Mongoloid. That’s why they are using those peoples as examples to begin with - they are separate identifiable groups.
This is an examination of the natural world and humanity as it exist - theres nothing about “white supremacy” here at all >


European would be caucasoid, sub Saharan is negroid and American Indian is Mongoloid. That’s why they are using those peoples as examples to begin with - they are separate identifiable groups.

They're just using groups that are distantly related, not representatives of three classes. Stop trying to backpedal here to a much weaker claim than the one you actually made. >

For tens of thousands to hundreds of thousands of years, there have been populations that reproduce predominantly within their own group.  Over time, their genes evolved, adapting to their environment, like your example about skin color.  But it's not only skin color, for example, difference "races" have different predisposition to getting different diseases.  Based on your genes, there is a lot of data about where your ancestors are from.