See Garmin Express Not Detecting Devices on Newer Mac OS if you are on Mac Catalina, Big Sur, or newer.
Ensure Garmin Express is Up to Date:
1. Open Garmin Express
2. Click Garmin Express in the Apple menu bar

3. Click About Garmin Express

4. Click Check for Updates to Express

5. Click Install Now if prompted

The latest version of Garmin Express can also be downloaded and installed from the Garmin Express page.
Garmin Express will be up to date once the installation completes.
If Your Device has Already Been Added to Garmin Express, Try Removing the Device and Adding it Back Again:
To Remove the Device:
1. Open Garmin Express
2. Hold Control (Ctrl) on your keyboard, and click the device when using a one-button mouse on a Mac

3. Click Remove from Express
4. Click Remove
The device will now be removed from Garmin Express
To Re-Add the Device:
1. Open Garmin Express
2. Connect the USB flash drive to the computer
3. Click Add Device

4. Click Add Device when the device is found

5. Follow the on-screen prompts to add the device to Garmin Express
Uninstall and Reinstall Garmin Express:
See Full Uninstall of Garmin Express
The latest version can be re-downloaded by visiting https://www.garmin.com/express once Garmin Express has been uninstalled.