If you receive the following message during a file transfer, it means that the remote Mac does not have sufficient permissions to access files and folders.
Starting with macOS Catalina (10.15), macOS asks for your permission when apps attempt to access restricted folders like Documents or Downloads. When TeamViewer tries to access one of these folders for the first time during a file transfer, macOS displays a prompt, asking for your permission to access the folder. This prompt is displayed on the remote Mac and blocks the app until the prompt is confirmed or denied.
To avoid situations where TeamViewer is unresponsive and unable to access the remote filesystem, with no one in front of the remote Mac able to confirm the macOS permission prompt(s), file transfers outside of remote control sessions require Full Disk Access permissions.
Required permissions for file transfer
Which permissions are required depends on how you are using file transfer: Standalone or during a remote control session.
Standalone file transfer (without a remote control session) requires Full Disk Access for the TeamViewer app.
File transfer during a remote control session requires
Full Disk Access or
Accessibility and Screen Recording permissions.
All permissions can be configured on the remote Mac through Help --> Check System Access in TeamViewer. You can also review and change permissions in System Preferences --> Security & Privacy --> Privacy.
If the remote Mac is managed by an MDM system, Full Disk Access permission can also be set via Privacy Preferences Policy Control (PPPC) payloads.