I've seen this problem reported many times with a Google search or in the bug tracker.
I saw the necessary file in this folder:
Kerbal Space Program\GameData\Squad\Tutorials\Hopper.craft
I copied it in this folder:
Kerbal Space Program\Ships\VAB
And then I could finally see the "Hopper" in the training mission.
EDIT: it's on version 1.12.3.3173 for Windows, on GOG
It's not accepted though, the "Next" remains disabled.