I have had the same issue for months now spending days try gin to sort it out. Spoke to HMRC today who advised me to re-set my VAT on-line account which we did. It STILL does not connect to quickbooks though! The HMRC advisor said I would have to contact Quickbooks. I did this last month and was told to contact HMRC. 

If this is not sorted out today I will have to close my business. Spent several unpaid days being sent from Quickbooks to HMRC back to Quickbooks and the Quickbooks still doesn't recognise MTD. It was fine since 2020 but has only recently become 'detached'. 

Can you help. I will need a refund for the last 3 months subscriptions as well as I am paying for a service that I am not getting.