Every time I get this message when an update is available.
I have looked at the logs and found that this was the issue:
Looking for running process: C:\Program Files (x86)\Garmin\Express\CefSharp.BrowserSubprocess.exe
Unable to kill process. A 32 bit processes cannot access modules of a 64 bit process.
CustomAction StopExpressProcessesKey returned actual error code 1603 (note this may not be 100% accurate if translation happened inside sandbox)
When digging a bit deeper I found that L-Connect (software for my cpu cooler and fans) is the source of this issue. When I closed everything related to L-Connect 3 I was able to install Garmin Express again. It would be great if this is resolved in the installer. I don't know why this process needs to be closed