The expected output is a single string with each row separated with line break.
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Oh.... Ok I got it. Thanks

You wrote the answer in the question. 6=1, 12=2, 18=3 ... Try to figure out the logic of these equations

In the example, input 6 = 1 line full of ■,
input 12 = 2 ■ lines,
input 18 = 3 ■ lines.
Why? Is there any special rules for those ■ lines?

At first I solved this kata pretty fast, so I tried to shorten my solution to 1 line with logic operators.
Then I gave up after struggling for over 3 hours. This is really hard..

ok nevermind I solved it, this was really simple and I thought the question overcomplicated.

We cannot help you without more elements. We don't know what you are actually doing. This kata has been solved more than 10000 times in C#, so it's likely to be an error from your code. There may be differences between the implementation of your IDE and the one on CodeWars but that's not a kata issue: issues are bugs in the kata. I advice you to read this documentation: https://docs.codewars.com/training/troubleshooting/

thanks for the comments; specifications have now been added to the solution setup

Nevermind, I solved it!
for those who are stuck at free() error: you should use malloc when defining the return array.

ok I found out some working codes for C after suffering for 3 days, but when I test this code it works and crashes at the same time and shows this:

Test Crashed
Caught unexpected signal: 6
free(): invalid pointer

Deleting free makes my solution work, so maybe the "free" thing at test code is a problem but I can't figure out what should I do.
I'll post my whole code on reply if it is needed.
Can someone help?