rear speakers don't work on yamaha rx-v377 recever

You're noticing an issue that I've hated about computer audio for years!
But there are 2 possible solutions.

1st - The most common way to connect a computer with 5.1 surround sound is by using the headphone jack type connectors (3.5mm) which allows for 2 channels per wire. This means for real 5.1 from your computer you need a system with:
1. Front R and Front L
2. Rear R and Rear L
3. Center and Sub
So 3 connections on the computer side and 3 (or more) on the receiver side. BUT! many receivers don't have all of those separate inputs for surround (including yours).
The receivers that do not have separate inputs for each channel of the surround expect to get surround audio from a Coaxial, Fiber or HDMI cable.
This means they must use a digital encoding to send the audio all on 1 wire. Which would work fine and still does for DVD's.

BUT, computer games expect that you have the above setup (using separate wires for each channel set).

So why don't games ouput in the digitally encoded single cable surround?
because to convert the audio in real time would introduce sound lag without encoding hardware.

For many years I wasn't aware of any type of better solution for this issue.
But recently I learned about Dolby Digital Live.
Dolby Digital Live uses hardware to encode the 5.1 audio in real time.
This allows you to connect your receiver using 1 cable for the surround sound audio.

This external sound card is capable of hardware encoding for Dobly Digital Live:
https/us.creative.com/p/sound-blaster/sound-blaster-omni-surround-5-1

Bottom line: Test your current setup with a DVD in your PC. You might find that the surround sound works fine. I've read that some onboard Realtek audio chipsets have the ability to convert to dolby digital live but it takes plenty of setup/configuring to get it working.

Note: Your receiver doesn't need to have the Dolby Digital Live feature listed. It just needs to be able to decode regular Dolby Digital 5.1 which is on just about every receiver sold these days.

Final note: I agree with madmatt30 you should connect your audio out from the PC directly to the receiver. But what I said above is still valid. (If even a DVD won't give you surround then go ahead and connect the PC directly to the receiver and try again.)

Plug your PC directly to a receiver HDMI input rather than to the TV & returning using ARC.

Barely any TV's offer full Dolby pass through from external sources nowadays.

Do this...
Video out - (HDMI to the TV) (This will give you 4K @ 60Hz) (It's probably an issue with the receiver passing 4K at 60Hz but your right about the HDMI cable needing to be 2.0)
Audio out - (Fiber Optic (OR Coaxial) to the receiver)

This means you'll need to specify the audio out should use the Fiber Optic (OR Coaxial) cable because it will start by setting it to HDMI by default.

Whats your setup exactly ???
Gpu & motherboard mainly
You can split the gpu outputs ,one for audio,one for video.

Optical cannot pass true HD .
For that you need to use displayport or HDMI.

My advice , buy a displayport to HDMI cable .

Run display port to hdmi to TV for video
Use HDMI to HDMI on reciever for audio (how you have it now)

No, just any fiber optic cable used for audio will work.

"Optical cannot pass true HD"
Optical isn't even used for video and you don't need to provide video to a reciever (although you CAN) because they just play audio. The scenario I suggested doesn't send video to your receiver (at all) only to your TV.

I stand corrected. Optical will not carry Dolby TrueHD. (I wasn't aware of that limitation.)
Now I understand why madmatt30 suggested what he said earlier.

HDMI to the reciever for audio
Display port to TV (if your TV has Display Port, if not you should get a display port to HDMI adapter.)

Alternatively you could use 2 HDMI ports on your computer if your video card just happens to have 2 HDMI ports.

^ True HD is a Dolby audio standard !!
Which cannot be passed using optical or coax - only hdmi or display port.

Also hardly any TV's will pass through Dolby surround - as we've already seen with the op's initial post which was the problem to begin with.

Did you set up the receiver to test and balance the channels? They all worked then?
Does the TV pass 5.1 surround sound? Some don't. Check the TV audio menu.
TV won't pass Dolby TrueHD. Try PC to receiver via HDMI and receiver HDMI out to TV. That will pass it. Optical audio will pass 7.1 digital surround sound but not lossless audio such as Dolby TrueHD.
The front panel of the receiver usually displays what kind of input it's getting.
Try different surround settings in the receiver.

^@americanaudiophile - he has 5.1 running PC to receiver .
Then issue is it'll only enable him to select 4k @30htz using receiver pass through whereas PC to TV runs 4k 60htz.

It may be a cable , it maybe a limitation of the Yamaha - I'm not familiar with it myself.