Google Drive API Client (Python): Insufficient Permission for files().insert()

Files: insert requires authorization with at least one of the following scopes

https://www.googleapis.com/auth/drive
https://www.googleapis.com/auth/drive.file
https://www.googleapis.com/auth/drive.appdata
https://www.googleapis.com/auth/drive.apps.readonly

Check which scope you are authenticating with.

As advised by @DalmTo, the issue was an incorrect Auth scope in:

 flow_from_clientsecrets()

If modifying these scopes or permissions, try deleting your previously saved credentials related file or browser cache if any.