I also had an issue with the payment through Barclayscard app. 2019-12-31T00:00:00Z
Around this date the app has some bugs. A week before there was a maintenance during the weekend and the app became useless.
I was able to pay through web browser though but the window box with confirmation code was really small - not user friendly at all.
You might want to call Barclay card and see if they can take payment over the phone, sometimes that solves system issues. Also, email acceptances@monzo.com and tell them about the issue you are having so they can look into it.
You can also pay via bank transfer using your card number as the reference if you need to pay quickly. I generally pay via bank transfer from Monzo to my barclaycard each month.