Hear me out.
Supermarket very busy this morning. Queued with my basket for the self checkout, as usual. I prefer them and enjoy the lack of social interaction required. 3 of them were out of order so only 2 working. There was a couple using one and I think it was their first day on the planet. They were just staring, puzzled at the screen. Occasionally scanning something. Eventually complete their transaction then realise they need to buy bags. 5 people completed their transactions on the other checkout while they were doing theirs.
Blowing my own trumpet, but I'm a self service ninja. I think you should have a certain competency level, like with driving, before being allowed to use one. You should be given a basket of items, including some loose fruit and veg and bakery items, and a set time to complete the transaction in order to earn a licence.