If you receive an error when saving your MWS keys to your campaign, like "Access to Products.GetMatchingProductForId is denied" - or a similar error, please check the following:
In your Amazon Seller Central account -> confirm that your account is "Active". If you don't, this could mean you don't have a healthy account status or your monthly subscription cost is overdue.
In your Amazon Seller Central account -> click 'Settings' -> 'User Permissions'. Confirm that you have added Synccentric as an app developer and that Synccentric has proper permissions to make requests on your behalf. You may need to Renew Permissions for Synccentric.
Review the MWS keys you generated in your Amazon Seller Central account with the credentials you added to your Synccentric campaign. It's possible you added a key or seller ID to the wrong field in Synccentric.