Oracle 11gR2 Installation on Oracle linux 7 - Prerequisite condition failed for OS kernel parameter "semmni"
Having the correct value in sysctl.conf does not mean that is the current setting in effect system-wide.

You can check the current value in effect with:

sysctl -a | grep "kernel.sem"


If it differs from the one in the file configured, you need to actualize it with

sysctl -p


Or it will happen on the next reboot automatically.