At a client recently we had an Exchange 2016 Server that would not install the March 2020 Security update. The setup would start and then end with the error below after a few minutes:

Error 1642 generally implies that it cannot find the version installed. Even though we had Exchange 2016 CU15 installed (as per programs and features), there was a security update for Exchange 2016 CU14 listed. We removed the update and re-ran CU15 and after the installation and reboot we were able to install the KB above.

Hope it helps.

By edward