Can you try manually adding a GitHub username/password to the Jenkins credentials list? You can use a GitHub "personal access token" for the password. The "scan credentials" list seems to filter certain types of credentials (like SSH keys) out of the drop down list.

I manually added keys from the credentials page for the scope "global credentials unrestricted" and they do not shown up in the plugin.

Attaching images

Can you let me know if the drop down box is properly populated if you add a credential with the type "username and password" instead of the "secret text" type?

Did you try creating username/password credentials? (the password can be the token)

Same issue. Re-creating the credential as username/password worked. But it's very confusing ...

The Add button cannot currently be provided with the context to know what credential types are accepted by the select drop down.

Thus when you add credentials, there is nothing stopping you from adding an irrelevant type of credentials or even adding an irrelevant type into a credential domain that will not be matched.

The best that we can do is to prevent the select drop down from allowing you to select invalid credentials

This is a duplicate of JENKINS-40293