What does 'unindent does not match any outer indentation level' mean?

Answer 1

Check amount of tabs and spaces in each string of your code :)

Answer 2

Python knows which lines execute in the same context by their indentation depth.

It can't determine which block the 'less-indented' code goes to; all it knows is that you can't back out a half step without first taking a half-step in.

for i in range(9):
codecodecode
for j in range(9):
codecodecode
print("which 'for' do I belong to? An exception.")

...either it's accidentally too far left or not enough (in the 'out' direction)