It's in some mix of cannabis soil I had, with 30% added compost.
A tomato plant got the same mix and is doing just fine, this cucumber guy seems to be unhappy.
I checked the pH (mixed soil with some water and used my pH / EC meter) and it was around 6, so I thought it should not be nute lockout, cucumbers are supposed to be between 6-6,5?
I have no clue but I would guess it's iron, guessing from images I've seen, but I'm surprised how it could be lacking iron if a tomato is doing fine in the same soil.
Also whatever it is, how should I fix it?
Sweeping generalization: they need lots of water, but they also need to be planted in a space that gives them a sufficient root base such that they can go a few days without drying out.