Isle of Man sole traders do not use UK Self Assessment. Making Tax Digital does not apply to Isle of Man income tax.
You file with the Assessor of Income Tax, not HMRC. The return is due by 6 October, for a year that runs 6 April to 5 April. This page does not tell you how to register.
The Income Tax Division's GN 4 - Self-Employment is the Assessor's note on self-employment. It is not a UK Self Assessment guide.
UK bookkeeping software still estimates tax as if you were in the UK Self Assessment system. That estimate is the wrong one for the Island. Read Using QuickBooks or Xero on the Isle of Man, then try the Isle of Man tax calculator.