You want to add other request in transaction code STMS_IMPORT. Unfortunately, the Add selection field is greyed out.
Solution
There is a transport request in the import queue which has no target client set. You may need to set the target client first before you can add other request.
Click on the transport request and set the target client.
Alternatively, you can remove the transport request from the import queue.
Reference
1876563 – Import options greyed out (missing transport trucks)
