Adding Zoho Mail to ZOho One raises a deployment/region error

Adding Zoho Mail to ZOho One raises a deployment/region error

Good morning!

We have Zoho One, based in the EU region. I try to add Zoho Mail to Zoho One. I add our domain name as requested and get the error:

The domain which you have entered belongs to another deployment/region.

There are no other deployement or region ?! Where is the problem.

Thank you