CentOS 6 is extremely old and latest FastNetMon Community does not support it.
You may find CentOS deprecations notes here
You can use only FastNetMon Community 1.2.0 version it.
I would recommend upgrade to AlmaLinux 8 or switch to Ubuntu LTS 22.04 as these distributions offer all modern capabilities.
I recommend making backup for /etc/fastnetmon.conf before installation.
Hostgroups capability is extremely limited in community version and if you need multiple hostgroups I can recommend Advanced edition which has lots of improvement:
Free one month trial can be requested using this form:
Sincerely yours, Pavel Odintsov