Acethinker Music Recorder 1.2.6 is a standard type or old style audio recording app requiring a driver or drivers that allow recording the output headed for the speakers. Unfortunately many hardware manufacturers disable that ability because DRM. It may be possible to simply turn on that capability in the properties dialog for the output in Win10's Settings -> Sound -> Sound Control Panel. Or it may require updating/replacing the audio driver your device uses, or there may be a registry hack, depending on make & model, or you can try to find a 3rd party generic driver, or you can experiment running a jumper from the headphone jack to the microphone jack. In contrast you can use one of the audio recording apps that saves the incoming audio stream as-is -- several have been on GOTD in the past. The advantage of saving the stream is you don't lose quality or add noise [PC/laptops are an electrically noisy environment, and some of that noise will show up in your recordings]. The advantage of digitizing the audio signal headed for the speakers is that if you use an appropriate app, you can also use VST plugins to filter the audio before it's written to disk, removing noise & improving the quality of the recording. Sadly, while Acethinker Music Recorder 1.2.6 includes some utilities that may be useful, it doesn't accept VST plugins. Its use of Windows Media *may* also have a negative impact on quality compared to 3rd party alternatives.
Installation isn't terrible, but it is a bit messy when it comes to the registry, registering several plugins & file name extensions for various audio formats -- I recorded 1235 new entries in my Win11 VM. Besides the program's folder [~72MB] folders are added to Users\ [UserName]\ Roaming & Local\ Microsoft\ Windows Media\ .