Using mathutils outside of Blender I have installed mathutils 2.81.2 in my Python 3.8.5 install to use outside of Blender. However, nothing seems to work. E.g. I cannot create a unit matrix m = mathutils.Matrix() and find its components by subscripting it m[0][0]. I get TypeError: 'Matrix' object is not subscriptable.
Printing it simply returns the default &lt;mathutils.Matrix object at ...&gt;.
And trying to multiply two matrices using @ or a matrix with a vector gives a TypeError: unsupported operand type(s) for @: 'Matrix' and 'Matrix'.
Isn't mathutils supposed to work outside of Blender? After all, I can install it fine using pip.
By the way, I have also tried the same on Python 3.7.4 (which was the version used in Blender 2.81). I get the same problems there.
It is working for me. I think you have problem with installation.