Hi, @AlejoMac. It does appear that domain name is already linked to another site or Netlify account. There are two ways to resolve this.
First, if you have access to the Netlify account or site where this domain is already being used, please delete the other site or domain configuration there first (or ask that account owner to do so).
Once any DNS zones and site configurations for this domain are removed from the existing account, it will be possible to add it to your account.
Second, if you don’t have access to the other site or team settings, our Support team can remove the link between this domain name and the other Netlify site/team - provided you can verify ownership or control of the domain.
We have a guide in our Support Forums that explains how to create the verification record here:
If the verification TXT record is created, our support team will be happy to delete the configurations for this domain from the other account.
Please let us know if there are any questions.