This happens due to the GDR update. If you installed GDR security updates , you could not follow the CUs of SQL Server 2016. This is the product design.
I would suggest you to uninstall the GDR update in window control panel. Then, install the newest CU17. CU packages are, in my opinion, better choice for most users. With newest CU17, your implementers should not have further problems.
See similar issue in here : Can't upgrade SQL Server 2016 to latest CU update.