If you have already tried adding a domain that you own to your GoHighlevel, added the DNS records provided by GHL, but the domain still can't be added, try the following:
1. Go to your Domain Registrar or Hosting and get to the DNS section of the domain
2. Check all the A records assigned to the domain
3. If there are other A records installed to the domain that you're trying to add to GHL, make a copy of those records and paste them somewhere where you can go back to them if needed. Then go ahead and delete those A records so only the one provided by GHL remains.
Important note: Make sure that those A records are assigned to the domain that you're trying to add to GHL, not a subdomain. If it's a subdomain that you're adding to GHL, then delete the other A records assigned to that subdomain except from the one provided by GHL.