[2020] Metro Line tool not working

Those are custom metro station assets, which I think is where the problem is. When I hold Shift I can place stops just fine, which indicates that the stations are missing some internal spawners.

Spawn Point Fix should solve that, but looks like it isn't for some reason. So you might want to either change the metro stations to another building that works better or just live with the fact that you have to hold Shift to add stops to those.