Microsoft sound mapper - Input no longer picking up audio & error -9999

I am totally new to Audacity and audio editing. I have been tasked with digitizing some old tapes and with some googling I figured out a solution: I would use a simple aux cord to connect a walkman to my laptop, and in Audacity I changed the input device to Microsoft Sound Mapper - Input. I also disabled the mic on the laptop so it would not pick up both. Then I was able to play the tape and Audacity would pick up the audio and record it. Simple. I’m now working on a different laptop, which as far as I can tell is running the same version of Windows 10 and Audacity (3.1.0), and the same Realtek sound system, but Audacity is no longer registering any audio, and if I try to disable the microphone I get error -9999. I’ve tried everything I can think of to fix it but I’m stuck. Can anyone suggest something that might make this work the way I expect it to?

That’s the down side of the simple, affordable AUX cable. The actual audio interface is inside the computer—and they’re all different.

I’ve been using a Behringer UCA-202 USB interface.

It has no volume controls (other than the one for the headphones) but it’s simple and does the job most of the time. Here it is connected between my analog sound mixer and my Windows machine.

My older Mac laptops had an actual stereo recording connection (circle with two black arrows) and I used that for recording production without an external interface for years

Not any more.

We should wait for a Windows elf, there may be a setting or configuration you can set to make this work.

What happened to the old laptop? I think Windows desktop machines still have a full set of connections. The stereo recording connection is usually blue. The microphone connection is pink.