Travel posting (Transaction PRFI) - errors (ECC6)

I am having a number of problems with posting of travel expenses to FI.

No 1 - message 'Trip has no postable amounts' when Per Diem for Accommodation is ticked.The Travel Expense Statement shows the Per Diem but has 0 under Cost Assignment and no Cost center information. If an advance is then given, the picture changes and Cost Assignment has valie and cost center details.

No 2 But then.... config set up to post to employee as Customer, the customer record exists, but now an error message is receved that the Customer is not found.

No 3. So then... changed config to post to employee as Vendor, the vendor record has been created but as with customer, the message "No vendor found for this selection'. Has anyone else had any of these problems and what is the solution?