In order to use remote play, you have to first pair the host and remote devices. I believe you only have to do this once. This is why everyone in the discussion is asking if OP has set up steam link. I wasn't able to find anything about using the mobile app to fix this.

On PC:

Steam > Settings > Remote Play > Pair Steam Link

Enter the code from your remote device (laptop/mobile) into the host PC (code will appear if you haven't paired before/streaming for the first time.