MyBB Community Forums

How to block the forum for guests and give them a message that they need to register?

1.
admin panel >> users & groups >> groups >> guests --> (Edit User Group) --> Forums and Posts --> Viewing Options

untick below and save user group
Can view threads?
Can search forums?
Can view user profiles?

2.
admin panel >> templates >> active theme templates >> Header Templates >> header_welcomeblock_guest

you can add message for the guest in above template.

or you can use guests can't view threads & Floating Guest Notify plugins

Thank you very much