I've been in touch with apple dev support on this, and I've just informed the issue has been fixed. I'm not sure if this was just for my particular case, or if a more general problem was fixed, but you guys might want to try and see if the endpoint works for you now. If it still doesn't, I'd recommend contacting dev support. Took a couple weeks, but the issue did get resolved.

I am having the same issue - I've verified that both accounts involved in the transfer meet all transfer criteria, and that the app itself meets the criteria listed here: https://help.apple.com/app-store-connect/#/devaf27784ff Yet I still cannot generate transfer identifiers. Any insight would be appreciated!