Python类型错误:“NoneType”对象不是可订阅的

I am trying to write a function that performs backsubstitution on a matrix already in echelon form, but every time I try to access an index of my matrix I get - TypeError: 'NoneType' object is not subscriptable. I've been working with this for hours now and am getting really frustrated, although I am probably overlooking an obvious detail. Here is my code: I am trying to write a function that performs b