The vendor may not be an approved vendor. Navigate to Vendor search in PeopleSoft - Main Menu > Vendors > Vendor Information > Add/Update > Vendor.
Check the "Status" of the vendor on the Summary Tab. This will determine of they are an approved or unapproved vendor.
A Supplier Information Form (SIF) may need to be filled out and sent over to DTS for processing.
For more information visit Disbursements & Travel Services