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.