I have a Shmeppy account that allows me to make maps but when I go to the testing site I can’t sign in. It says my email isn’t registered
The testing site is a completely isolated copy of Shmeppy so it doesn’t know about your account or subscription on the main site.
If you want to start using the testing site, make a player account on https://testing.shmeppy.com/ using the same email that you use on the main site and then email email@example.com from that email asking to merge your subscriptions.
Only your subscription will be merged, the two sites are still completely independent. So games won’t be shared for example.