For some reason there are instances where the customer account got corrupted and their unique number was change to zero. You will know when this happens when you see the customer account name on other invoice and PO with no relation to that account at all and you won't be able to edit that account as well.
To fix this you need to log in to the training data and check the customer account there and take note of the unique number as shown in the image below.
Connect to the customer server and access the Pervasive Control Center press win+R then type in pcc. Collapse Engines>Server name>databases>live dsn name>tables double click on Account see image below.
Then type this in select * from "Account" where AUnique = 0 then press F9 the customer in question will now show at the bottom result
Then change the AUnique number of the account to the unique number you see in training for that customer in this example the Unique number of this customer is 10. Click anywhere above to save then you should be able to edit that customer in the live dataset.