The bin/scripts directory for you Python installation isn’t on your PATH.
I think this isn’t unusual, since by default the installer for windows doesn’t add the interpreter bin directory to your PATH (however there’s a checkbox you can click in the installer if you want this).
Instead of relying on python/pip being on your PATH, it’s recommended to use the py launcher, as you did in the first command.
Apparently, if I preface it with: py -m it works fine (in my other pc, this was not the case). Before, I could just type pip install library_name. Now, I have to type py -m pip install library_name.