Xero treats Contacts in a specific way: a Contact only qualifies as a Supplier when there is at least one Bill associated with this Contact.
There are 3 ways of dealing with this:
Use a simple workaround: create a fake Bill for the Contact and then delete it right away. Now, this Contact will be treated as a Supplier by Xero and you'll see it in ApprovalMax.
Please note: If you don't see a Contact right away, please sync Contacts to ApprovalMax.
Another option is to allow Requesters to see All Contacts in ApprovalMax, not only those marked as Suppliers. This is configured in the Requester matrix:
Provide Requesters with the ability to create new Contacts, if this is required.