Cannot add subdomain

[daffoml]

Ive got 1 domain that has a subdomain, and I'd like to add a second, but the button is greyed out. I have no idea where to even start to look for this problem.

Unfortunately the owner is not available for me to delete the subdomain and recreate it, so I cannot try that route at this point.

I can create multiple subdomains in every other domain I have tried (3 of them).

Any suggestions on where to start?

Thanks