This might be caused by a large numbers of routes assigned for a location.
For example, if client is looking to add a US route for a Tulsa Sales Order, a very large number of routes will appear in the results, leading client to believe that the desired route is not available:
:
To solve this, client simply has to press F4 on the above screen, and then click on the arrow pointed below, this will bring up a menu where they can narrow down the results. So, if in this case client is only looking for routes for the US, they have to type US* in order to get all the routes available for the US: