Unable to install macOS Sierra on iMac 2013

That may actually be a hopeful sign. That particular error code simply means the Mac cannot establish a secure connection to Apple's servers. There can be a variety of reasons. Try a different Wi-Fi network, Ethernet... etc. In rare cases resetting NVRAM has been reported to work: Reset NVRAM on your Mac - Apple Support

If your Mac starts up to error -1008F - Apple Support describes a similar error condition. If that Mac has been "activation locked" as described in that document though, it's a show-stopper. There is no circumventing it... not unless the Apple Account holder deactivates it for that Mac.

Internet Recovery (⌘ ⌥ R) does not require a Recovery partition, so something else is going on. I have no idea what "ChromeOS Flex" is or what it does, so it is possible installing it corrupted the EFI Firmware required to load Internet Recovery, in which case you're out of luck with that Mac.

You could try installing a Linux variant such as Linux Mint: https://linuxmint-installation-guide.readthedocs.io/en/latest/

Be advised I have other Linux installations on Macs but not that particular one.

Great! Thanks for the update.