It appears that Garmin Express can not update itself. This issue is showing up with V184.108.40.206 and macOS 10.15.3
I use the Forerunner 910XT on MacBook Pro macOS Catalina (10.15.2). Some time ago (on an older Mac machine) I used Garmin Training Center with Garmin Ant Stick and Garmin ANT Agent, but this no longer works. At the MBP I have no USB connection, so I can't use the charging cable or the Garmin Ant Stick. Garmin Support Center United Kingdom is where you will find answers to frequently asked questions and resources to help with all of your Garmin products.
A ticket has been opened at Garmin, they are working on this.
However, given the symptoms of this issue, Mac users will most likely have to go to Garmin and get a new copy of GE until GE can update itself again.
So I went and opened GE to check if anything new was available for my devices and if a new version of GE was available. I noticed right away that GE was not indicating that GE was up to date, and I had to manually initiate an update check. That query resulted in a failure message. This all started around Friday, so I let it go as Garmin doing some sort of system/server maintenance. The issue is still here on Monday. I called Garmin and they confirmed the problem on one of their Macs. We checked to make sure that my Mac was set up correctly, and it is.
Download Garmin Express for Mac to keep your nuvi, zumo, or dezl up-to-date. Use Garmin Express to update maps and software, sync with Garmin Connect™ and register your device. This desktop software notifies you when updates are available and helps you install them. DOWNLOAD FOR MAC DOWNLOAD FOR WINDOWS. Dec 16, 2020 Fixed compatibility issues with MacOS Catalina Changes made from version 4.3.1 to 4.3.2: Fixed issue with detecting Garmin-mode USB devices. Dropped support for Mac OS 10.10, 10.11 and 10.12. Changes made from version 4.2.0 to 4.3.1: Update to 64-bit app. Fixed issue that caused map installation to fail. Changes made from version 4.1.1 to 4.2.0.
GE was updated Jan 21, 2020 to V220.127.116.11, and macOS just a couple days later. This problem showed up after the macOS update.
I'll post here when the issue is resolved, or any updates.