Amazon has a known bug in their Advertising API that affects sellers who used the same email address for multiple Seller Central marketplaces, for example SC Europe and SC North America.
There is no danger in having set up your Seller Central accounts in that way, but we will be unable to pull advertising data from one of the marketplaces.
However, there is a workaround. The following is the suggested solution by Amazon's tech support.
Follow this 5 step guide to get your advertising data working in Seller Metrix.
Log in to the Seller Central account that you currently cannot sync Advertising data for.
Go to Settings -> User Permissions.
Under "Add a New Seller Central User", add a new user (using a different email, for example api_auth_eu@your-comapny.com). Optional: You can "Manage Permissions" for this user and it's enough to give it "View & Edit" permission to your Advertising data.
Your will now receive an email-invite to create a new Seller Central account for api_auth_eu@your-comapny.com. Set it up as an individual account (free). This new Seller Central account will now have access to the advertising data for your Seller Central account.
Use your new Seller Central account when connecting your Advertising account to Seller Metrix.