You need to grant M2E Pro access to your eBay account to maintain the inventory and order synchronization with eBay marketplace. Please go to eBay > Configuration > Accounts > eBay Account > General and click Get Token. After you are redirected to the eBay website, sign in to your seller account, then click Agree to generate a new access token.

After the token is obtained, please Save your Account Configuration in M2E Pro.