Either no recording on Mac or Loud noise on Windows

I just purchased a Retround USB cassette capture, but cannot get the software to work. First I started on my PC. The first tape was fine. When I tried to do the next one, I got a very loud screaming sound. I have spent hours looking at the Help and the Forum, but in spite of being in the computer business for many years, I don’t understand the language of sound. I did find a posting on the Forum saying it could be that from recording and playing at the same time, but I do not understand what to do to solve the problem.

Then I tried my Mac. There, the problem was only getting a straight line and not recording the sound from the machine. I found a workaround, but even after I followed the instructions in the terminal, I did not get any results. Again, there must be a setting that is wrong, but I have no idea what it is.

It is very frustrating to not be able to speak with someone about this, so I hope that in this forum I will get specific instructions about how to set things up on the Audacity app for either the Mac running Catalina or the PC running Windows 10.

We see a lot of problems with these devices. They are made (very cheaply) in China and re-badged as various “brands”, none of which are better than others.

Regarding the squealing sound - that is probably due to “feedback” through the built-in microphone.
“Feedback” occurs when sound from the microphone is amplified through the system, plays out through the speakers, and is then picked up again (louder) by the microphone - the sound is “feeding back” from the speakers into the mic. The sound rapidly builds up to a squeal or howl.

You’re probably thinking “I’m not using the mic”.
You would be right that the mic “shouldn’t” be active, but if you’re getting feedback, then it is. This could happen if the USB cassette loses it’s connection.

Check that the USB connector is secure, and if possible, try a different USB lead.
Then check that the USB device is enabled in the Windows / macOS Sound settings.

When you’ve confirmed the above is OK, either restart Audacity, or tell Audacity to “Rescan Audio Devices” (near the bottom of the “Transport” menu).
Then ensure that the USB cassette is selected as the recording device in the “Device Toolbar” (see: Device Toolbar - Audacity Manual)

You may also find this page of the manual useful: https://manual.audacityteam.org/man/recording_with_usb_turntables.html