I've added my Constant Contact API key to my Big Commerce store, it verified the API but it's not syncing or pulling up my newsletter lists. Error message on this page is attached for your reference.
The store log indicates: HTTP Status 401 - Application data not found for key: 8b82ab59-4a4f-4dfc-a350-706f3a2b85d9 %lsantoski
I did not put the key here for security reasons.
Big Commerce supports their integration to us, so I would contact them if you still have issues, but make sure you're using the Big Commerce API Key listed in Step 5 and that there isn't a space before or after the key.
Partner API Support Engineer