yes, you will see the error becuase the server is running with 12. RU1 schema and you might be adding different version of SEPM as a load balance.
You can add a new server as a load balance with 12.1 RU1 version and then upgrade both the servers to RU6.
OR
upgrade the existing environment to 12. RU6 and then add a new server.
check this article
https://support.symantec.com/en_US/article.TECH178820.html