Post Custom Command does not run anymore

CustomCommands are run in the context of service. If your google drive mapped to a user account then it’s not available to a service (like any other mapped drive).

See the topic for information on mapped drives:

The other way would be to set:

PostCustomCommandUISession=true

This tells MCEBuddy to run the custom command from the currently logged on users UI session, in which case it will have access to the mapped drives for the logged on user