This is a community forum, you’re unlikely to get a response from anyone who works at Monzo on here. Your best bet is probably to email them at help@monzo.com
No clue what your friends and family are doing differently but the list of ineligible transaction types for Flex is at Monzo Help - Eligible transactions for Flex and includes financial services companies and credit cards. There’s also a Reddit post at Reddit - The heart of the internet that says it used to work but now doesn’t.
Generally all credit cards have a block on using them to pay other credit card bills unless you’re specifically have a balance transfer option on the card. Not all card have that (or offer it to all customers) and it’s generally a one-one ‘move everything on that card to the other card’ arrangement.
I don’t think any credit card will let you pay a regular monthly other credit card bill with it (or setup a direct debit). Or else you’d be able to setup a round-robin of just using a chain of cards to pay each other and never actually paying any ‘fresh’ money into the system…
Remember, Flex is a credit card. You can’t use it for other credit services which Klarna falls into.
I could be wrong, but like Paypal, Klarna would use 2 different payment services:
One for one-off purchases (instant payment required)
One for “Buy now, pay later”
Klarna should allow credit cards for one-off purchases with instant payment, but anything classed as credit (buy now pay later, pay in 3, pay in 12, etc) wouldn’t be allowed. However, it does all depend on how the system is set up.
We accept major credit cards such as Visa, Maestro, and Mastercard. Please note you cannot use credit cards to pay off purchases made using Klarna’s credit products, including the Klarna card.
btw - if you are doing this and relying on it to ‘balance’ your outgoings, please talk to someone about this as this can (and almost definitely will) spiral out of control.
Someone like Stepchange to review what you’re doing and help you out.