There’s another thread all about this but basically lots of Post Offices, especially those that are in Corner Shops like to ‘double-dip’ on your transactions by instead of using your Debit Card exactly as such, they like to try and do a LINK ATM withdrawal instead.
That means they get commission for doing whatever transaction you are doing in the Post Office AND for a LINK ATM withdrawal. Because Monzo aren’t part of the LINK Network this doesn’t work and they throw their toys out of the pram.
Insist they process it as a proper Debit Card transaction and everything will be OK.
EDIT: Just realised its this exact same thread so I suggest you read the rest of the thread and others on this topic!