This error can occur if one of the Fields on the Supplier record are not set up correctly, for example the Controller. If you look at the Fields, that are populated and there is one that has no narrative text next to it that will be the one with the issue.
To correct:
From the Safe Toolbar go into the Purchase Ledger
Go into the Suppliers
Locate the Supplier you need to update
Navigate to the Supplier tab and then the Main sub tab
On the field that is populate but does not show the narrative text ,click on the ellipsis button.
This will then give you the list of options available for selection
If the one that you are attempting to use does not show, you need to Right Mouse Click, Create Record, populate the necessary fields and the Apply to save.
If it shows it maybe that the details are not fully complete, Right Mouse Click, Modify Record, populate the necessary fields and the Apply to save.