Not sure if this error "Invalid registrant information" is caused by WHMCS or ENOM WHMCS module, I think rather WHMCS.

When client enter wrong format of lets say ZIP number in his profile, he should be notified about the fact. Because domain registrations (on Enom for example) fails if example ZIP code has wrong format. And customer then complains domain was not registered.

Discover cause of error:

Go to WHMCS administrationa area, go to Utilities menu, then Logs submenu, then click "Module log". In module log, search (Ctrl + F) for the error: "Invalid registrant information" (without quotations marks)

If you cant find on latest page, you will need to go to Next page, and so on. untill found.

I found this loglines:

Err1=Invalid Registrant information.
Err2=Zipcode was invalid for that country. Sample format: 5419LJ
Err3=Invalid Aux Billing information.
Err4=Zipcode was invalid for that country. Sample format: 5419LJ
so the client entered ZIP code format that WHMCS/ENOM do not like. Example working format is: "5419LJ"

Fixing issue

One can
a) contact client to change the field and suggest valid format
b) go to client profile ("Profile" tab) and update his/her ZIP code or other invalid field.

Then retry domain registration. If some error arise, check Module log again.