Then looks at the section titled: "Windows WASAPI loopback recording "
Note that you need
a) the Host to be WASAPI
b) the Recording Device to be … (loopback)
You may find you need to stretch out the Device Toolbar sto see the …(loopback)
Hover over the little vertical bar at the end of the Tools Toolbar and drag it rightwards.
Update: and I note that the 2.1.2 Manual says this is for "On Windows Vista, Windows 7 or Windows 8 only " - ignore that, it was written before Windows 10 came out. WASAPI works fine on Windows 10 (I use WASAPI/loopback all the time on my W10 laptop. I corrected the Manual for the upcoming 2.1.3 version a while back - where it now says “On Windows Vista or later …”).
It is best not to use abbreviations in your posts. What is “comp radio” that you said you wanted to record?
You need to decide which device you will use to play the audio, for example computer speakers. Are you listening on computer speakers?
When you have decided what playback device you will use, the Audacity playback device must be set to that device, and the Audacity recording device must be set to (loopback) of your chosen playback device.
In Audacity, ensure Transport > Software Playthrough is OFF (not ticked, not checked).
In the first (Host) box of Device Toolbar, choose “Windows WASAPI”.
In the second (Recording Device) box, choose the (loopback) input for the device you are using for playback. For example, if you are using “Speakers” for listening to playback, choose “Speakers (loopback)” in this step.
In the third (Recording Channels) box, choose mono or stereo.
In the fourth (Playback Device) box, choose the Speakers or Headphones that you are using for listening.
Start playing the audio you want to record.
Press the big red Record button in Audacity.
If you still get the error after that, you may have to change the Audacity project rate bottom left, or change the Windows sample rate of your playback device, or both.
Why 8000 Hz? That will make it sound like music played down a landline telephone. If you want that effect you should do it in the Equalization effect after recording. Better to set 44100 Hz project rate then it will have more chance of matching with the Windows Default Format Sample Rate for the speakers as set in Windows Sound.
If necessary try going into Windows Sound and looking what the Hz rate is set to for the Speakers. Then set Audacity Project Rate to the same rate, with the two Exclusive Mode boxes underneath Default Format in Windows Sound unchecked (no tick mark). Restart Audacity.
If that does not work, put the tick marks back for Exclusive Mode and restart Audacity.
Is this when recording with Windows WASAPI loopback? If so, that can happen on some systems. The “system mixer” is Windows Sound but you can’t use that to adjust the WASAPI loopback recording level because WASAPI loopback does not appear as a recording device in Windows Sound. Instead, try adjusting the playback level in Winamp or in whatever application is playing the sound.
If adjusting the output level in the application does not affect the recorded level it doesn’t really matter because WASAPI loopback should then record at a fixed, moderate level. If the level is not loud enough, you can Effect > Amplify… it after recording.