Once a domain removal is started, there is no way to undo it and mark the domain as 'healthy'. The DNS zone in O365 is gone (if your domain had full delegation), some users and groups on domains are moved to the initial domain, there may be other things in AAD partially deleted etc.)
There is a flag in AAD for the domain that indicates the forced removal failure, and can't be cleared up - you now have to complete the domain removal.
Follow the instructions in the domain details page for the domain – manually delete any references still using the domain (users/groups/apps/shared mailboxes/etc) that prevented automatic removal, then complete the domain removal.
If you want the domain back on tenant after that, you can add it back later and re-configure it.