This Knowledge base article helps with identifying the Operating System details on which NSX appliances [Manager and Edge] are running.
VMware NSX
VMware NSX-T Data Center
Perform the following steps to check the underlying Operating System details:
1. Login to appliance as root user.
2. Run any of the following commands to get the OS details:
a. cat /etc/os-release
b. lsb_release -a
c. cat /etc/lsb-release
d. hostnamectl