In order to rectify this, we need to check if the MSP still has access to the end customer's tenant. Please follow the below steps to check this:
1. Sign in to the Partner Center dashboard and select the Customers tile.
2. Enter the customer's name, domain name, or Microsoft ID to search for the customer.
If you don’t see your customer on the customer list, You need to request a reseller relationship.
3. If you see the customer in the list check if you see the license information.
4. Open Microsoft 365 in the Administer services list.
5. Please check if the license information is available here.
These are the few checks we need to do if the error continues post these checks please contact email@example.com