Hello,
Are you able to logon to the default demo company via direct link and then able to delete the faulty company? E.g. direct url:
businesscentral.dynamics.com/<guid>>/?company=CRONUS%20International%20Ltd.
...where company=CRONUS%20International%20Ltd. should be replaced by your actual demo company...
If this does not help, best is to contact your partner or Cloud Solution Provider and ask them to open a SR (via Partnersource) with Microsoft Support on your behalf for the best support experience.