Error: Connection activation failed: Device not managed by NetworkManager or unavailable

If you run ifconfig -a or ip link show as root, does the eth0 interface appear in the output? Does it have the MAC address that ís specified in the /etc/sysconfig/network-scripts/ifcfg-eth0 file on the HWADDR= line?

If it is not there at all, then the driver for the NIC has probably not been loaded yet, or has a problem that prevents it from loading. If it has an unexpected name, you may have a problem with NIC naming.

If the NIC is there with the expected name, but refuses to activate, it might be a firmware loading problem: some NICs require firmware to work, and if the necessary firmware file is not available, the driver cannot activate the NIC. If this is the problem, then the dmesg | less listing will usually contain one or more messages indicating that the system cannot load a specific firmware file. Once you know the name of the firmware file, you can search for it. Make sure the linux-firmware-<YYYYMMDD>-<version number> RPM package is installed: it includes a large collection of firmware files for various hardware.

system-config-network

select the eth0 device and hit the Edit button and uncheck the "Controlled by network manager" check box and check the box for Activating the port when the computer starts up. Save and quit in there.

Now run system-config-services and stop/disable NetworkManager and start/enabled network service.

This works for me