Conflicting ownership with other DNS zones

I want to add my externally registered domain to Netlify DNS, but it’s giving me the following error:

name is already taken, heartbits.pt is conflicting the ownership with other existing zones

As mentioned, the domain is heartbits.pt
A friend was previously using it on his account, but that deployment has already been removed

What can I do?

Thanks in advance!

EDIT: I have performed the steps to verify ownership as stated here

Hey @cesium,
Thanks for doing the legwork ahead of time! I was able to verify domain ownership, so you should now be able to use the domain on your site. Please let us know if you run into this issue again.