@CharleyTheCoder Netlify need to remove it from the other account manually.
To do this they need you to verify that you own the domain by following the instructions here:
https://answers.netlify.com/t/support-guide-verifying-domain-ownership-for-netlify-custom-domain/13765
The value you would use is:
https://answers.netlify.com/t/deleted-my-account-and-caant-connect-my-domain/128736
Once that’s done, you post here to say it’s ready and then you just wait.