@strictlysocialnz It sounds like another plugin is resulting in this critical error. If you wish to once again use the Health Check & Troubleshooting plugin you can re-enable other plugins once my one, checking for the same issue each time. You can then also re-active your WordPress theme while in troubleshooting mode, it’s possible there may be theme functions or customizations conflicting with plugins your site setup.
As the issue may also be server related you can also perform a search on troubleshooting such issues, or checking the Fixing WordPress forums, with topics such as this one describing how to check your error logs.
Let me know if you need assistance checking your plugins using the Health Check & Troubleshooting plugin. You’ve successfully checked Site Kit using the same plugin so it’s just a case of performing more checks with your other plugins.