you keep incrementing the str pointer. When you return it, it is pointing to the nul-terminator, so it's effectively an empty string. Also, you overflow the char datatype by adding 13 to the last ASCII letters (e.g. 'z' + 13)

712,577,413,488,402,631,964,821,329 is bigger that the space for read using %ld Long int -> 2,147,483,647 https://en.wikipedia.org/wiki/C_data_types
you need work with parts of numbers using a string for keep the total

It's written in the description :

In C, the initial dimensions of the GoL universe is passed into your function by reference via the pointers rowptr and colptr.

Read this. Check how you calculate r, is n an int?

And how is your solution being wrong an issue?