Hi, @Kenolise 
welcome to the Netlify support forums!
This is happening because the custom domain you’re trying to use already exists on Netlify under a different account.
Please verify domain ownership and let us know by responding here when that’s done. This is what I see right now.
host -t txt verified-for-netlify.uncloudy.co
Host verified-for-netlify.uncloudy.co not found: 3(NXDOMAIN)
If the verification is successful we can remove the domain from the other account so you can use it on yours.