First of all, please mind that this kata uses terrible requirements for its return value, which goes against common C practices. It's not something what you'd do in any kind of C code.

What you need to do is to return a dynamically allocated array, where each element of such array is a pointer to an individually, dynamically allocated Pair:

Pair** result = malloc(sizeof(Pair*) * amount_of_returned_pairs);

result[0] = malloc(sizeof(Pair)); // individually allocate each Pair, WHY
result[0]->first = ...; // assign values to the pair
result[0]->snd = ...;
// ... and so on for remaining pairs

return result;

Don't ask why it's done like this, and not just in a normal way of allocating all pairs at once:

Pair* result = malloc(sizeof(Pair) * amount_of_returned_pairs);

result[0].first = ...; // assign values to the pair
result[0].snd = ...;

return result;

If you still can't get it to work, i'd suggest asking for help on #c channel of Codewars Discord (and from the start explain what kata you work on, because initially people might not realize how twisted requirements here are), solve it in a higher-level language, or just leave this kata, ignore for now, and come back to it when you get better grasp of memory management in C.