The "domain name already exists" error appears when the custom domain you are trying to connect already exists in the Kartra database. This means that it is already associated with a Kartra account.
If the domain was recently disconnected from Kartra (from your current account or a different one) and you are attempting to connect it to Kartra again, this could result in the error you're seeing. Please allow 24 hours from the time the domain was disconnected before trying again.