Hey 
I haven’t seen this particular error message before, but I’m guessing they’re referring to 3DS. The prepaid Monzo card doesn’t support 3DS so if a particular merchant requires all card payments to go through 3DS, they will fail any attempts with your Monzo card 
This is why the Monzo card doesn’t work with Coinbase for instance. Our next generation of cards will work with 3DS and they’ll arrive with the current account 
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