please elaborate how you are integrating your endpoint with pega by giving the Authentication details etc..
from 7.4 we have added a capability to generate the datamodel and required artifacts to successfully hit and get the token to fetch the response from App studio.
if you have already generated the datamodel using Dev Studio rest wizard and now the run time execution is failing to get the access token when used from Activity/Datapage. please try with Conneted apps landing page to get the token. (which uses just the Authentication profile - if that also fails please reconfirm the client details like id and secret for Client credentials grant type)
Please mention the full and proper details on how you are trying along with steps will give us the pointers to help you better.