Invalid reseller user id
Admin reseller-codes API rejected the user id (400 Invalid reseller user id.).
Why this happens
Empty/malformed id, or not a real user on this site. Not a member-facing reseller API key error.
Diagnose and fix
1. Open the user from Admin → Users (do not paste arbitrary ids).
2. Confirm the account exists and is marked reseller if required.
3. Retry the reseller-codes action from the user context.
4. See Reseller codes (admin) and Approve reseller on register.