It’s possible that this is the difference between accessing a cash machine with the LINK app (which will be on the Barclays card, but not on the Monzo one) versus accessing a cash machine with a Mastercard/Visa app. In the early days of Monzo there were significant messaging issues on ATMs because the Monzo card doesn’t support LINK:
They also often only offered much higher withdrawal amounts by default:
And forced you to get a receipt with your withdrawal:
All this is to say, that it very well might be a ‘Monzo’ issue in that Monzo cards don’t use LINK like the vast majority of bank cards in the UK. And this lack of a suitable message might be a remnant of the way Mastercard/Visa cards were treated by ATMs. Whatever software this one is running must not provide a suitable message for non-LINK cards.
The issue is more likely Not given the option to choose a different amount when note denomination is not available. I can see how problematic this would be, as when your card gets spat out, you have no way of knowing that it would work if you put it in and chose a different amount to withdraw.
I suspect this doesn’t apply to all ATMs. It would help if you could report who operates the ATM you tried to use. I wonder if @bamesjerry might be able to provide insight, as he dealt with the ATM messaging issues previously. Emailing acceptance@monzo.com might also be worthwhile.