There's a NaN error in one of the tables on my PHP Laravel Script, What could be the problem?

Question

This is an Inventory Management System with POS, HRM, and Accounting site. The code was working perfectly, But suddenly its began to display some errors, I edited and corrected them only to be stocked at NaN error in my add sales table.

NaN instead of the actual number.

Tried to debug, But I can't find the source of problem.