One reason Quicken won't add your new security is that the security name is already found on your security list. For example, the ticker "RITM" will add a security named "Rithm Capital Corp", but trying to add ticker "RITM-B" in Quicken after that will fail because it is also named "Rithm Capital Corp". The solution?
Temporarily change the assigned name of ticker "RITM" in Quicken and then add ticker "RITM-B". You may want to change the names of tickers "RITM" and "RITM-B" in Quicken after that.