Scientific Computing Sudoku solver

My program in step 15 of sudoku solver keeps giving error. Nothing’s working. What do I do?

If you’re getting an error you should read the error and see if it indicates what the problem could be.

If you’re asking for more help than that you’ll need to share:

The error
Your code
A link to the project you are working on

Any other relevant information, what you’ve already tried, your thoughts of possible solutions etc