[DocTor882]

Whenever I try to join the server it tells me “Failed to verify username!”

[Cil]

It sounds like you changed your username and the launcher needs to update, if that doesn't work I'm not sure

[TheDutyPaid]

There are quite a few things that can cause this, the server side is fine and no other reported issues.

Lots of help via search engines with this, as asking questions based on your set up and account type would be a full undertaking.

[thatguy3339]

Typically this happens if you change your account username, you will want to open the launcher and click the dropdown in the top left with your username in it, then click "Log Out"

After you do that, just log back in, and the issue should be fixed.