Why is there no conjunction between 健康 and 有活力的生活状态 in 无法保持健康有活力的生活状态? In the following sentence:


  那些不注意身体的人，到时候，无法保持健康有活力的生活状态，白白留下人生的遗憾，也让爱你的亲友感到痛苦。


The sentence above should mean something like the following:


  Those who don't care about their health cannot keep the healthy life, completely regret their life, and make their friends sad.


But I don't understand the grammatical structure of the bold part. Especially because:


保持 is a verb here and its object is 健康.
But then I don't understand why 有 follows after 健康. 有活力的生活状态 is also a noun, so why is there no conjunction between these two nouns?


So my questions are:


Is my understanding on the grammatical structure here correct?
If it is, then why is the conjunction omitted here? And is it common to omit the conjunction in Chinese and don't even use a comma (逗号)?
Your translation has the right idea. It reads something like this:


  Those who don't take care of their own physical well-being will be unable to maintain good health and vitality in their later years, leaving themselves with regret and causing pain to their friends and family.


The bold part can be made clearer, for example like one of the following:


無法保持健康、有活力的生活狀態
無法保持健康而有活力的生活狀態




The reason why it's not clear is because it is a vernacular/spoken sentence written down literally (which you can tell because the sentence begins with「那些」; formal literature does not do this). It is discouraged to write like this unless someone is being quoted verbatim, because such sentences require more effort to interpret when read.

During transcription, the necessary conjunction (or enumeration comma) was simply left out. If you hear the original person saying this, however, they might have inserted a pause, or dipped the tone slightly, between 健康 and 有活力, making interpretation much clearer.