может быть ниже 4 причин для этого типа ошибки, которую вы нашли ..
1. You have your PayPal module configured to "Sandbox" but have not used Sandbox API credentials.
2. You have your PayPal module configured to "Live" but have not used Live API credentials.
3. You have mis-typed one or more API credentials entries, including but not limited to leaving trailing spaces.
4. You have deleted and/or recreated new API credentials in your PayPal account but have not copied those new credentials back to your store.
Просто проверьте ... это может быть полезно для вас ..
Спасибо.