The other ship in the fleet is the same class so It cant be that. What is a commercial engine?
When you design engines, you can specify that they are either military or civilian. Civilian drives are larger, and there are other differences. The main issue is that a ship with any military systems - which are large sensors, weapons, and military drives, require maintenance and can only utilize military jump drives. Ships with only civilian systems don't require maintenance or overhauls, and can only jump using jump gates or civilian jump drives.
I don't see any other obvious military systems on your ship, so you must be using military engines. Design new engines and look at the drop down boxes on the engine design sequence. I believe the bottom one is the one where you can select military or civilian, or fighter or gunboats drives, if you've researched those. Select civilian drives, and then create the system. Do an instant development on the new drive system, and then replace the military drives on your ship with your new civilian drives. The resulting design should be able to jump, as long as the resulting design is smaller than the installed jump drive capacity.
Kurt