I don’t understand your connections, therefore pic to show how the cables and adapter are connected.

What is the C adapter - the USB-C end of the Sonos Line-In adapter?

And what happens when you just connect the Sonos Line-In adapter to the Era 100 with no source? 

I’m still not sure what you mean here, are you using a line-level-output, or an amplified headphone output? If your Headphones work I assume that’s with a Headphone amplifier (if using line-level output from the PC?) .. I’m just confused by your answers, so perhaps a photo or two will help, as suggested by ​@Mr. T earlier.

USB C is C adapter.
3.5 from PC to the back or Era100 direct.
3.5 from PC to C adapter to Era100.
PC to BT works fine.
 

You have to use the Sonos Line-in adapter, a direct connection to Era 100 will not work

3.5 from PC to the back or Era100 direct.
3.5 from PC to C adapter to Era100.

If that is how things are connected, the Line-in should show as connected in the Sonos app.

Are you able to confirm audio is being output from the 3.5 on PC?

3.5 to Era100 = https://www.sonos.com/en-us/shop/sonos-ace-aux-cable-black
3.5 to C = https://www.sonos.com/en-gb/shop/sonos-line-in-adapter-black
Both of them do not allow me to in line it.

To confirm, your Era 100 is either standalone or in a stereo pair and not used as a surround?

Purely connecting either the Ace aux cable or the Sonos Line-in adapter to the Era 100 should then show a Line-in connection as active in the Sonos app. If that doesn’t occur, then you may have an issue with the USB-C port on your Era 100.

With all due respect. Stop replying. You have no idea what you are doing or talking about.

To confirm, to confirm, to confirm some more?

The Era100 is an Era100. Suround, Around the world, Over the hill, it makes no difference.
The 2 connections I have showed numerous times are the 2 I have tried (Above & here -
3.5 to Era100 = https://www.sonos.com/en-us/shop/sonos-ace-aux-cable-black
3.5 to C = https://www.sonos.com/en-gb/shop/sonos-line-in-adapter-black
BT (Bluetooth) works fine however I want it hard wiring.

The Sonos app for the final time shows nothing for line in………...

With all due respect, I can get the Era 100 Line-in working with both the Ace aux cable and the Line-in adapter, but I’ll leave you now to work it out for yourself.

With all due respect, I can get the Era 100 Line-in working with both the Ace aux cable and the Line-in adapter, but I’ll leave you now to work it out for yourself.

 

If only you read the first post……………. PC to Era100.

Actually, in your first post you said “Just got an era100 for line in to PC’

The USB-C port is an Input only on the Era 100, it is not a sound Output

Here’s a recording of (literally) ‘anything’ connecting to the line-in of an Era-100 (called Noshery R in my setup here) - just an RCA audio cable only will automatically make it’s line-in active - you don’t need anything connected at the other end of the audio link, whether it’s a PC or any other audio source. The line-in should become active in any case.

If it isn’t doing that, then there’s likely something wrong with the audio cable, the adapter. or the speaker.

Hence my earlier suggestion to try any other audio source, just  to see if it will work/play audio… if not then there’s likely a fault with the audio cable, the adapter, or the speaker.

It’s definitely working okay here for my Era 100. See the line-in become active within a few seconds when a connection is made just with a 3.5mm audio cable to the Sonos Audio Adapter plugged into the back of the speaker.

I have tried 3 cables in total and nothing detected. Bluetooth is working fine.

Thats wrong. A USB C is just a smaller USB that has both flow and the ability to charge. Please stop typing. You and your turntable hurt my head.

Keep trying the impossible.

It may help if you understand the function of the USB-C port on the Era 100.

Not sure if this is relevant here, but just in case. There are several different types of 3.5mm male jacks on audio cables - see the attached. The ones I use here are the middle TRS jacks shown in the photo for stereo audio - is that what you’re using too?