Actually my environment is like below:
Windows Server 2012 R2 Standard - master server NBU 8.1.2 (we already scheduled update ASAP)
RHEL 7.5 - media server NBU 8.1.2
I need before did update my master server add new media server with specifications NBU 8.1.2, OS RHEL 8.5.
My basically doubt if is there any problem add new media server with different operation systems?
Different OS for media servers is not a problem in Netbackup, you can mix and match Windows & Linux/UNIX. The only requirement is that the operating systems are listed in the Software Compatibility List.
And you may want to bookmark the Documentation landing page : https://www.veritas.com/support/en_US/article.100040093
As mentioned by @Nicolai , there is no problem to mix different OS types in a netbackup architecture all what you have to do is to make sure of NBU version compatibility and your OS.
and one more thing, for RH 8.X you will need to check this prerequisites first : https://www.veritas.com/support/en_US/article.100045645