What can we do in this case ? customer is not aware who or why used OTD previously - how to take ownership back ?
I'm just another user - but I have spent some time with OTD registration issues. It's tied very closely to 365 authentication and whether you are a Global Admin or not. Hope this helps.
Please open a support ticket with Poly on this issue. Most likely the domain your customer is attempting to add was incorrectly added to a different tenant, which can happen when organizations have multiple M365 tenants (production, dev, test, etc.)
From a user's perspective - I can add on to Jeff's comment that there are some really talented Poly people who understand 365 Tenants and OTD and Azure and can get into the weeds and help you figure this out. You just need to navigate your way through support to get to them.