If a new employee is unable to select or change the counterparty (seller) in a document, the issue is most likely not with the document itself, but with that user's access rights.
What's the issue
The "Seller" field in the document header is filled in automatically according to the value of the "Seller" field in the tabular section. If the employee doesn't have sufficient rights, the system may not grant access to editing certain document fields — this creates the impression that the counterparty "can't be selected."
What to do
You need to check the new employee's access rights in the system — specifically, access to editing the document fields where the counterparty/seller is displayed. If the rights are insufficient, they need to be expanded so the user can edit the required document fields.
Frequently Asked Questions
Why can't the "Seller" field be changed manually?
Because it is pulled from the value specified in the tabular section of the document — this is normal system behavior, not an error.
How can you tell if it's really about access rights?
If another user with full access can work correctly with the same document, then the problem really is with the new employee's permission settings.