how to fix this problem (screenshot)

Nothing special.

This only notices you of the fact that there was one or more brute force attacks against your server. From some ip's and with some usernames.
Normally nothing to worry about.

However, if you don't know what this is, I wonder if you did enough on server security.
I would at least install configserver firewall (csf/lfd).
And there are a lot more things you can do (search the forums, read the docs about it or hire somebody to secure it better). ;)
 
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