Board Game Arena

See title. I click the unit. It shows an arrow towards the transport, but nothing happens if I click.
What do I need to do? Or is it not possible in the first move? The handbook does not say anything against it and the example turn even shows exactly that move.

That's exactly as you're supposed to do it, click the unit then click the transport. It's possible on the first move, eg Naples infantry to the Ionian Sea fleet.

Are you on mobile? You have to click the ship piece and not the water around it and it can be small.