I'm trying to use the / hat command on my server but there is an error message:
hatfail: You must have something to wear in your hand.
I have permission to that command and op on the server.
The command doesn't work on any item: e.g. Helmet, Zombie head, On any block or item, etc..
Normal players who dont have op has permission to use command /hat can put everything on its head.

Please search for duplicate issues before creating your own. This issue is a duplicate of #2763, which contains a fix for your issue.