hello, i have been having the same issues with some RT booster missions not being able to finish.
it is because of updated parts that are replaced in the game.
the old parts that where used can still be used though.
for example the rt05 and rt10 booster engines you can find them in the parts list (when you click the arrows in the left upper corner there is a filter)
you can then choose an older version of the rt5 or rt10 booster engines.
if you use that one it will work.
the same method can be used for a bug when you play a tutorial and there is no engine to choose while building a rocket.
this problem should really be solved in a final update. the game is really nice but these bugs are really trashy,
