As they say, a picture is worth a thousand words.
Moonlight, your image shows you're in a Balao.....that's important to know.
Take the FOTRSU v0.40 Beta mod out of "Activation" within JSGME, using the path......Data/Submarine/NSS_Balao/open the NSS_Balao.upc file with Notepad. Drilling down to the CrewWatch area of the file, you'll find something like this:
In the 2nd Watch grouping, notice the problem with the "CrewMemberSlot_AttackWC2" parameter. The parameter is missing which "Watch" designation it belongs to.
It should read like this:
One little typing error is all it takes!!
After correcting the error, make sure you "Save" your file by using the Notepad "File" menu heading.
Start a new Campaign, or at least be in Port when you make the .upc file change. You should have the third slot of the 2nd Watch Crew available now.