“There is already an active certificate for the selected domain name, that is not issued by Let’s Encrypt. If you would like to switch it to Let’s Encrypt you have to first cancel it. Please contact our technical support if you need assistance with this.”
This is not a message from Let’s Encrypt. Presumably they’re referring to the technical support of whatever site or software you were using at the time while trying to get a certificate. I’m not entirely sure why a certificate issued by another CA would matter to them, but they should be able to answer that if you reach out to their support.
Let’s Encrypt doesn’t care about certificates issued by other CAs.