You may notice when trying to use the External Thread command you can't select certain circular edges. This is because the External Thread command requires a solid cylindrical body, and not a hollow feature, in order to work. You can create hollow features with external threads via the following sequence of operations:
Applying external threads to a hollow cylindrical part:
Note how in the below example, when using the External Thread command, the outside circular edge can't be selected:
Find and delete or suppress the feature that hollows the cylinder out, or modify the first sketch so your extrusion creates a solid body. Once your cylindrical feature is solid, note how the circular edge can appropriately be selected, and the External Thread applied:
After threads are successfully applied, now you can hollow the part out with a new feature to achieve your desired end result.