Oh, I didn't realize the blue dashed line. Ok, I have a better idea of how convoys work now. I really wish there was a better tutorial.
[attachment=0]Screenshot from 2023-07-13 08-56-35.png[/attachment]

I was under the impression it was using the fleet in ENG as a convoy. Is there another step?

Wales is not adjacent to NTH therefore the convoy requires the participation of the Fleet in ENG, which you did not order.

From your screenshot it seems you ordered F NTH convoy A Wal Bel, which is redundant but legal; and instead forgot to enter the necessary order, ie F ENG convoy A Wal Bel

... from WAL to BEL ?
(sorry)

Was F ENG given the order to convoy from GAS to BEL ?

I thought I understood convoys, but I keep having situations like this where an army can't move, but I don't see a reason why. In this case it has support and no opposition. Can you not attack via convoy even unopposed?

[attachment=0]Screenshot from 2023-07-13 06-25-27.png[/attachment]