What does 'You are not allowed to give reputation to this user' mean?

I have found that sometimes when I "like" a post I will receive that error message.

 

Usually it is a glitch or double-click... if you re-check the post you will probably find that you have "liked" it.

If you've already liked it, you can't like it again, but you can unlike it. Reckon op's issue would more likely be a case of user they are trying to like has a restriction (suspension), or perhaps a glitch using a touchscreen and holding too long, or summat.