Plans to provide this service were paused back in 2018.
See this link: https://monzo.com/blog/2018/06/21/paying-cheques-into-monzo/
You can however post cheques in, as detailed here: https://monzo.com/help/payments-getting-started/cheque-how-to-pay
I’ve paid a few cheques in via post and it was a very smooth process, your notified the moment they receive it and given the date the cheque will clear