For my Sherman tank I have almost completely fixed the turret except for one part. In status it says for interior turret I only have one part missing, one of the two listed commander seats. But when I click on the missing part it says I can't put anything more in assembly mode. I took everything apart and put it back again, but the bug still occurred. I did that again but after restarting the game and it still occurred. I think part of it may be that I used crafted parts, or that since the tank was excavated the parts were put in the wrong way.