Cannot obtain a token (authentication error)
How to isolate the cause when you cannot obtain an access token. Explains the representative errors — invalid_client, invalid_grant, redirect_uri_mismatch — and what to do about them.
403/401 is returned (permissions and scopes)
How to isolate the cause when an API call returns 401 or 403. Explains the typical cases: invalid token, insufficient scope, no store access permission, and User-scope review not passed.
app_not_approved is returned for User-scope calls
What to check when using a User-scope results in an app_not_approved error. Explains development with a sandbox slot, applying for review, and the flow from sandbox to production.
Webhook is not being delivered
How to isolate the cause when a Webhook does not reach your receiving endpoint. Walks through checking, in order, the endpoint settings, subscribed events, reachability, signature-verification failures, and firewalls.