Hi @VickiMe & welcome 
Several layers of the Monzo onion need peeling here…
Plus (and Premium) paid tiers only apply to Personal accounts. Joint accounts are excluded from paid tiers - a whole other subject
You can pay from pots (aka ‘Bills pots’) using a Personal account (no Plus/Premium paid tier needed) or using a Joint account - which is what we do.
If you have a Joint account, you don’t need a paid tier unless the benefits of the paid tier apply to your Personal account only.
Plus/Premium being available only on Personal accounts has always clashed with users who operate with a Joint account only - like me/us - increase the pressure to sort this out here: Monzo Plus and Premium (2020) for Joint Accounts
Do you have a Joint account? From the post title I’m guessing so - but Plus/Premium doesn’t (yet) translate to Joint accounts - only Personal accounts. Painful.