If your Garmin Forerunner 255 Music is not syncing with your iPhone, here what you can try:
Make sure that your Forerunner 255 Music is connected to your iPhone via Bluetooth. On your iPhone, go to Settings > Bluetooth and make sure that your Forerunner is listed as a paired device.
Check that the Garmin Connect app on your iPhone is up to date. Open the App Store on your iPhone and check for any available updates for the Garmin Connect app.
Restart your iPhone and Forerunner 255 Music. Make sure to turn off both devices completely, and then turn them back on.
Make sure that your Forerunner 255 Music and iPhone are within range of each other, and that there are no obstructions that may be interfering with the Bluetooth signal.
Try forgetting the Bluetooth connection between your iPhone and Forerunner 255 Music, and then re-pairing them. To do this, go to Settings > Bluetooth on your iPhone, find your Forerunner, and tap the "i" icon. Then, tap "Forget this Device" and confirm. Afterward, pair your Forerunner with your iPhone again.
If none of the above steps work, try uninstalling and then reinstalling the Garmin Connect app on your iPhone.
