You need to grant M2E Pro access to your Amazon account to maintain the inventory and order synchronization with Amazon marketplace.
In your M2E Pro, please go to Amazon > Configuration > Accounts > Edit Account > General and click Get Access Data. After you are redirected to the Amazon website, sign in to your seller account and grant Extension an access token. After the token is obtained, please Save your Account Configuration in M2E Pro.
If the error persists, please go to your Amazon Seller Central account and Revoke access token from M2E Pro. Then, complete the steps above to authorize M2E Pro.