Hey @MrWhiskins, sorry you ran into this issue while trying to create a joint account, we have just fixed this issue this morning, and will be released in the next week or so.
To fix the issue now, you need to ensure you have ‘Payments with friends’ enabled, which can be found on the Account tab, under the settings menu for your card. We also require access to the contacts on your phone to match them up with Monzo users. This can be enabled on the Payments tab 
Hope this helps you fix the problem, let me know if not.