Your solution is an impure function which does not actually fit the criteria for this kata. You need to create an (ideally pure) function which solves the sudoku and returns the solution, rather than mutating the input and returning True.

This is why the error message reads:

Incorrect solution for the following puzzle: [[5, 3, ...]]: True should equal [[5, 3, ...]]
                                             Notice this => ^^^^                           

Pro tip: When you copy and paste solutions from the internet, it helps to understand what it is you're copying...

Not a kata issue, resolving.