Test Mode not working

When enabling Test environment in Enom, under Domain Mapping > Reseller options, and trying to purchase a domain in the subsite, under Tools > Domain Mapping > Purchase Domain.

I'm getting the following error:
See image 1

When I switch to a Production Environment, it seems to work:
See image 2

Trying to test this in test environment, before going live. Seems like Test mode isn't working.