Boardgent remote desktop's file transfer needs "Full Hard Disk Access" to navigate the folders on your Mac. To enable permissions you must follow the next steps:
On your Mac open the Apple menu and select System Preferences, click on Security & Privacy, and then Full Disk Access.
In Mac OS Monterrey and earlier, in the left side bottom corner, you will find a lock, if it is closed you can click on it to unlock the changes.
Turn on the checkbox next to the Boardgent application.
Note: At any time in the process your Mac can ask you for your password to enable these permissions.
Mac OS Monterrey and earlier:
Mac OS Ventura and later: