@MikeyB A bit late but positive, When you are obtaining the invalid grant error as a result of same motive like me Then you certainly clear up it by resetting the secret important from the google console and go with the token generation stream once more with the new solution https://www.reddit.com/user/Consistent-Load1428/comments/1n7bz6p/herbal_medicines_online_cureayu_nonprescription/