This will be the regional settings on your device not matching with the UK-centric date format which Monzo requires - have you tried setting the language of your device to UK (usually ‘English, United Kingdom’) to perform the set-up? You can probably revert back to your preferred language setting after setting Monzo up.
Also, while you don’t have to be in the UK to open an account, a UK address is needed to send the Monzo card to. From the Monzo website;
"How to get a bank account without proof of address
When you open a bank account with Monzo, we don’t ask you for proof of address because we verify your date of birth instead.
What do I need?
To open an account, you just need one of the following:
- Passport
- Driving licence (provisional is fine)
- National ID card
- Other government issued photo ID
And you’ll also need an address in the UK where we can send your debit card."