Can't map a domain to a subsite set in multidomain

Hello dear support staff.
I'm creating a multisite network on
My issue is that after adding another domain ( as selectable in the multidomain, tried to map the same domain to but I can't.

The error I get is that ( it is an invalid domain.

Is there some way to use the same domain as multidomain and map it to a subsite?

Thank you for your help!

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