User is trying to add a vendor as a resource in the Services and Expenses tabs of a fee schedule, but cannot find the vendor in the drop-down.
This happens because, by default, Core only lists vendors of type Contract Employee and Outside Consultant in the Resource drop-down. If you want the vendor to be displayed in the drop-down, you can change its type to one of these two. However, if this is not feasible, you can choose to show all vendor types in the drop-down by selecting the Include vendors in employee drop-downs option in User Preferences > General screen. Check out this article to learn more about being unable to add vendors in budgets.