PRTG can't use AFP shares, only standard SMB-based shares. That would require creating a custom-script sensor that checks the "status" of the share and reports that status into PRTG according to our Custom Sensor's API definition. For more information, see PRTG Manual: Custom Sensors.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.