Hello Steve,
So there are numberous apps to transfer the Audio Only from a Youtube video. I know this because I have used most of them. And most of them work well. But since this is a sketchy operation it is not unusual for all of these to fail for one reason or another at one time or another. As a result, after years of hit or miss, I settled in on what I find to be the most reliable for me.
I use Audacity, a free PC Recording application. What it does is simply record to audio what ever you are sending to your PC speakers. So, first I configure Audacity to record my Speaker output, then I arm the Audacity track. Hit record on the Audacity Track and immediatley hit play in YouTube. After all is done. I then save the recorded Audacity track as mp3 or .wav or whatever audio format I care for. Drag and drop that into Reaper and all is good. If there are any audio glitches, I simply repeat the process.
The only challange with this is the Audacity configuration can be tricky at first. So I have written down exactly what I need to do. Fact is if I only use Audacity for this single purpose, I just set it and forget it. That is until I have an update and all settings go back to default.