Your URL is incorrect. You need to insert the USER id, not the email ID.
According to the Google API documentation,
1. Lists the drafts in the user’s mailbox
GET https://gmail.googleapis.com/gmail/v1/users/{userId}/drafts
2. Gets the specified draft
GET https://gmail.googleapis.com/gmail/v1/users/{userId}/drafts/{id}
Proof
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!