Hello,
In your case, it is important to have the SEP 12.1 RU2 client package imported on the SEPM 12.1 RU1 version.
I am not sure if you have imported the SEP 12.1RU2 package on SEPM 12.1 RU1.
In order to import the SEP 12.1 RU2 package, you may need the Symantec Endpoint Protection 12.1. RU2 DVD for which you may need to contact Symantec Technical Support as this is now not available on :
https://symantec.flexnetoperations.com/control/symc/registeranonymouslicensetoken
and once you have the Symantec Endpoint Protection 12.1. RU2 DVD, you would have to follow the steps provided in the article below:
How to manually import client packages into Endpoint Protection Manager (SEPM)
http://www.symantec.com/docs/TECH122824
I would recommend you to download the Symantec Endpoint Protection 12.1. RU3 DVD instead and migrate the SEPM to latest version of SEPM 12.1 RU3 which may assist you installing SEP 12.1 RU3 clients.
Hope that helps!!