As listed on https://community.monzo.com/t/merchants-banks-who-dont-yet-recognise-the-monzo-current-account-sort-code-debit-card-open-wiki/13891
IG is not currently accepting neither Monzo current account debit card, nor current account sort-code / account numbers. I've tried this over the weekend, and it is still true.
Could you please update your BIN database to include Monzo debit card, as debit cards?
Could you please update your sort-code validator to include 04-00-04 as Monzo Bank Limited, branch Monzo?
It is a normal UK GBP current account with fasterpayment, BACS, direct debit and CHAPS. Enter 04-00-04 into http://www.fasterpayments.org.uk/consumers/sort-code-checker to verify this independently.