I recently installed Mass Action Scheduler in a sandbox. I went to the Authentication Setup Wizard (Mass Action Auth. Setup tab) and the Automated Setup button no longer works because it tries to create a Connected App, which Salesforce no longer allows. So, I followed the manual instructions in GitHub and created all specified metadata: Auth Provider, Named Credential, and swapped out an External Client App for the Connected App steps. However, when I go to create a new Mass Action Configuration, I get an error saying that it can't find the Named Credential (API name of my credential is Mass_Action). I am a System Admin and I have the Mass Action Admin permission set assigned. What am I missing?
It sounds like the package isn't recognizing the Named Credential you created.
I'd double-check that the API name matches exactly what Mass Action Scheduler expects and that the Named Credential is active and saved properly.
Since Salesforce changed how Connected Apps work, it's also possible the package hasn't been fully updated for the new External Client App setup.
If everything looks correct, I'd check the package's GitHub issues or documentation to see if there are any known issues with the latest Salesforce changes.