Oh, I've just checked on my own computer and I'm now seeing that same message. (It wasn't there the last time I used it). Sorry about that!
Either save into MP4 format (at 720p) and then use VLC for the conversion, as per my previous post, or try one of the countless online services. I've just given this one a go and it worked for me:
https://ytmp3.cc/
(One small note though: After the file has been converted you'll see two download buttons. The big black one didn't work when I tried it but the smaller blue one did the trick).
Apologies again for directing you to something which didn't work!