Try booting the Mac into safe mode.
Safe mode (sometimes called safe boot) is a way to start up your Mac
so that it performs certain checks and prevents some software from
automatically loading or opening.
- Start or restart your Mac, then immediately press and hold the Shift key. The Apple logo appears on your display. If you don't see
the Apple logo, learn what to do.
- Release the Shift key when you see the login window. If your startup disk is encrypted with FileVault, you might be asked to log in
twice: once to unlock the startup disk, and again to log in to the
Test to see if Bluetooth can connect while in safe mode. If it can, restart your Mac without pressing any keys during startup for the regular boot.
If you are still having issues connecting, make a new user account (with admin privileges) and see if the problem exists on that account.
If it exists on the other user account then you may need to reinstall MacOS Mojave.