Make sure its USB mode is set to MTP mode, not Garmin mode.

Watch dmesg to see when the device is connected and ready:

[Fri Jun 28 15:27:59 2024] usb 1-2.1: new high-speed USB device number 36 using xhci_hcd
[Fri Jun 28 15:27:59 2024] usb 1-2.1: New USB device found, idVendor=091e, idProduct=CENSORED, bcdDevice= 0.01
[Fri Jun 28 15:27:59 2024] usb 1-2.1: New USB device strings: Mfr=0, Product=0, SerialNumber=5
[Fri Jun 28 15:27:59 2024] usb 1-2.1: SerialNumber: CENSORED

Use lsusb to verify:

Bus 001 Device 036: ID 091e:CENSORED Garmin International

android-file-transfer/aft-mtp-mount will not enable to you to write files on to it, instead it will show your device does not have android EditObject extension, you will not be able to write into individual files if you mount with it and creating files will lead to cannot create regular file errors.

Use simple-mtpfs instead (via).

simple-mtpfs --list-devices should list
1: GarminFenix 6S Pro/Sapphire

Mount it: simple-mtpfs /tmp/mnt

Copy something: cp -v some_file /tmp/mnt/GARMIN/

Unmount it: fusermount -u /tmp/mnt

Wait for the clock to be ready.

Done!