'NO' should equal 'YES'

This test case expectation is for array above the printed text (not below). The one you showed expects 'NO'.

Also, if you open a new thread, please continue there, don't create new ones for no good reason.

Please see if this helps: https://docs.codewars.com/training/troubleshooting/#print-input

the error message is most probably caused by the fact that OP's solution has a side effect (it mutates global variables), and is called twice. First call returns a correct result, but its used only to build an assertion message. the next call uses mutated state and returns invalid result, assertion fails, and the message composed in the previous step (i.e. the one with the correct answer) is displayed.

Test suite is wrong because it calls user solution twice. Solution is wrong because it uses global variables.

Well, I'm not sure how you got that error message, because for me your code produces other problems, like

solution.c:13:10: error: use of undeclared identifier 'first_numn'; did you mean 'first_num'?

because of the syntax issue of apparently intending to show multiplication of first_num * n instead. This also happens at 2i which should be 2 * i. Finally, you also have variables declared outside the function that should be inside the function. That might be it, just don't forget the include the inttypes header.

Please see here on how to post code in kata discourse: https://docs.codewars.com/references/markdown/#code-block

Hi @RJ20192, can you please post your code with proper markdown and a spoiler tag? I have a guess as to why your code might fail but I'd like to see why you're getting that error message, thanks.

Which language?

The last bullet point of this paragraph could be helpful: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution#when-i-print-my-answer-it-looks-exactly-the-same-as-the-expected-output-yet-tests-fail