Azure AD Connect not manually syncing

My recommendation would be to do one of two things:

1. Create a Support Request with Microsoft Azure Support (Identity Team). Submit with the new case a zipped folder from the following path, C:\Program Data\AADConect. Zip that as it contains Trace Logs and Installation Logs from the latest install of the Software which is giving you the issues with the PS Module. They can help troubleshoot why this is happening and resolve the issue once and for all.

2. If you are not worried about the root cause of the issue. Follow the guidance of the other reply(ies), and uninstall reinstall Azure AD Connect. This should resolve the issue. If it doesn't follow Step 1 for sure.