It’s got a guitar/line switch so it should be good!
I would be best if you get a left/right “splitter” cable/adapter, leaving one of the Walkman’s channels unconnected. With the wrong adapter going from stereo-to mono, one of the channels may be shorted to ground or the left & right may be shorted together and the sound could be damaged, and there’s even a small chance of damaging the Walkman. You probably won’t find the perfect adapter so you may need a combination of cables, maybe something with RCA connectors in-between.
I’ve got a couple more comments (and I’ll look for a link to some adapters if you need that) but I gotta’ go offline for an hour or so…
…I’m back.
Just to clarify - The Walkman is stereo but with a mono tape the left & right audio will be the same. You can connect to either one, but not both, since the interface has only one line-input.
Your interface is “stereo” with mic on the left & guitar/line on the right. (You don’t want to use the mic input.)
This combination of cables or equivalent will work.
This Adapter plugs into the Walkman.
This Adapter plugs into the interface.
An RCA Cable in-between.
Note - There is a “quirk” when you use only one input on a stereo interface…
If you record in stereo (both Windows and Audacity set to stereo), of course the left-microphone channel will be silent. After recording, you can delete the silent channel to make a mono file and it will play out of both speakers.
If you record in mono (both Windows and Audacity set to mono) the signals will be cut in half (-6dB) so they don’t go over 0dB or clip when the mic & guitar are combined. With the microphone channel unused, the peaks can’t go over -6dB (50%). You can amplify after recording, but you generally want to leave some headroom (to prevent accidental clipping) and amplify later anyway.
Digital recording levels are not critical as long as you don’t “try” to go over 0dB. If you remember analog tape, you wanted a hot signal to overcome tape noise. But with digital, no tape noise! And you can digitally amplify losslessly after recording. Also, analog tape was more forgiving as you went over 0dB where it began to soft-clip (or “saturate”). Digital is absolutely hard-limited to 0dB and the analog-to-digital converter (inside the interface) will hard-clip if you try to go over.
…If you ever want to digitize a stereo tape, you’ll need a different interface with 2 line inputs.