It looks like this is not an issue with the Ansible Tower plugin specifically as it appears to happen with other plugins. i.e. the gitlab plugin. Please do some testing and let me know if you agree that this may be a larger problem.
John Westcott this does not seem to be an broader issue since I was able to create a credential from the add button when trying to configure both Gitlab and Github servers (see the attached screenshots). When I try to do the same with the Tower plugin nothing happens when clicking on the add button, that dropdown is never shown nor is the new credential modal.