When I create/save "Template" jobs in Ansible AAP, the "credential" field is not enforced by default and users/I tend to leave it blank. but when running the same Template, AAP throws this error at the first task in a playbook:

fatal: [<hostname>]: UNREACHABLE! => {"changed": false, "msg": Failed to connect to the host via ssh: Warning Permanently added 'hostname, IP' (ECDSA) to the list of known hosts"

So, it should make "credential" field required in the Job template by default.