If a repository has a commit, how can I trigger it?
Have you tried the GitHub Trigger node? I suspect the Push event might have what you are after.
Problem running workflow
Check that the repository exists and that you have permission to create the webhooks this node requires
That is then a problem with the token. Either you did not give it the required permissions to do so, or your user does not have the permission to do that in the first place. You can for example not use it on a repository you do not own or you are not the admin of (the last part I am not 100% sure how exactly it works but something like that). Best test with a repository your user owns. If it does not work there, then the permissions of the token are the problem.