Welcome to Monzo 
It’s unfortunate that your card didn’t work in the Pay at Pump 
Morrisons is my go-to for petrol (normally the cheapest in my area 
) and I always use Pay at Pump 
I’ll have to see if it works for me when I next fill up 
There’s a thread that cropped up last week: Morrisons Supermarket Petrol Pay at Pump not Working
Looks like you’re not the only one having a problem! 
Hopefully Monzo are able to look into this, if it’s becoming more than just a one-off