You are trying to add a domain on Acquia Cloud and receive the error:
An unknown error occurred. Please try your request again. If the problem persists, Contact Acquia Support. Request Id: xxxx
This can occur if you have another Acquia Cloud application where the domain is already in use. A domain can only be used for one application at a time.
You will need to remove the domain associated with the other application before adding it to the desired application on Acquia Cloud. This process is typically quick and, by setting a higher caching time or utilizing a CDN, the impact to visitors can be reduced.