My forum URL is https://yumdm.flarum.cloud/
I am having trouble with a couple of people signing up to my forum - they are getting the following message:

They have both tried two emails, and all four emails get the same message. Is this a setting I have installed, and is it something I can do anything about?
Sorry if this is more of a Flarum question - I wasn’t sure.
TIA
Russ