user-1: Sometimes, when flagging an answer, this message appears:

Attention: some of your recent flags have been declined - please
  review them before flagging this post!

What I am supposed to do? I follow the link to the review, but I can't see any action, button, link or nothing.

user10794: There is no particular action to take. I got this message a while back when I was going through the review queues and flagging posts. If the (then) moderators didn't agree I would eventually accrue enough "flag declined" messages that the "system" (Stack Exchange) started automatically warning me.
I think the warnings eventually expire through time.
You might consider doing less flagging of things that should (could?) be handled by community moderation. That is, just vote an answer down if you don't like it. Or, if you don't want to take the reputation hit, post a comment explaining how the answer could be improved, or explain why you think it is wrong.
See my Meta post about not flagging answers as "low quality" just because they are a bit short or you think they are wrong.
Flagging as "not an answer" should be reserved for answers that are simply not answers. Like, gibberish, spam, not in English, or asking another question.
Even link-only answers are marginal, they are answers after all, just not good answers.

user220: You can actually view a list of your flags and how they were handled if you would like to review. If you go to your user profile on the Activity tab, you can click on the XX Helpful Flags and you get a list of all of your flags.
I don't know exactly how the algorithm for warning works, as roughly 90% of your flags were deemed "helpful." If you're worried about it, look at your declined posts to look for a pattern (in your case there does seem to be a specific type of flag).
Also, when looking at your flag history, please note that Disputed is not Declined. Here's what it means: https://meta.stackexchange.com/a/95277/190823

user359: It's supposed to make you aware that you have been flagging incorrectly and that way teach you what is considered a good flag and what isn't.
There is no real actions to take on the website except to flag better.

-1: <p>Sometimes, when flagging an answer, this message appears:</p>

<blockquote>
  <p>Attention: some of your recent flags have been declined - please
  review them before flagging this post!</p>
</blockquote>

<p>What I am supposed to do? I follow the link to the review, but I can't see any action, button, link or nothing.</p>
 10794: <p>There is no particular action to take. I got this message a while back when I was going through the review queues and flagging posts. If the (then) moderators didn't agree I would eventually accrue enough "flag declined" messages that the "system" (Stack Exchange) started automatically warning me.</p>

<p>I think the warnings eventually expire through time.</p>

<p>You might consider doing less flagging of things that should (could?) be handled by community moderation. That is, just vote an answer down if you don't like it. Or, if you don't want to take the reputation hit, post a comment explaining how the answer could be improved, or explain why you think it is wrong.</p>

<p>See my <a href="https://arduino.meta.stackexchange.com/questions/2430/please-dont-flag-as-very-low-quality-answers-you-just-dont-agree-with">Meta post</a> about not flagging answers as "low quality" just because they are a bit short or you think they are wrong.</p>

<p>Flagging as "not an answer" should be reserved for answers that are simply <strong>not answers</strong>. Like, gibberish, spam, not in English, or asking another question.</p>

<p>Even link-only answers are marginal, they are answers after all, just not good answers.</p>
 220: <p>You can actually view a list of your flags and how they were handled if you would like to review. If you go to your user profile on the <code>Activity</code> tab, you can click on the <code>XX Helpful Flags</code> and you get a list of all of your flags.</p>

<p>I don't know exactly how the algorithm for warning works, as roughly 90% of your flags were deemed "helpful." If you're worried about it, look at your declined posts to look for a pattern (in your case there does seem to be a specific type of flag).</p>

<p>Also, when looking at your flag history, please note that <code>Disputed</code> is not <code>Declined</code>. Here's what it means: <a href="https://meta.stackexchange.com/a/95277/190823">https://meta.stackexchange.com/a/95277/190823</a></p>
 359: <p>It's supposed to make you aware that you have been flagging incorrectly and that way teach you what is considered a good flag and what isn't.</p>

<p>There is no real actions to take on the website except to flag better.</p>
