You have recently upgraded the Symantec Endpoint Protection Manager (SEPM) and you are using AutoUpgrade to upgrade the client packages.
If you go on the client group > right-click on a client > select Edit properties > On the General tab you see “Deployment Message: The client has ignored upgrade package, version 12.1.xxxx.xxxx”
“Deployment Message: The client has ignored upgrade package, version 12.1.xxxx.xxxx”