Если не является простым, то как раз и должно быть False, ещё раз почитайте условие задания

I cannot speak to your two previous experiences, but in this case your code does not work for this kata, yet. Notice that your code passes the Sample Tests? That's because it is basically correct. Meanwhile, it times out for the Final Tests because it is not optimized enough to solve all the tests in sufficient time. This then speaks to the fact that your outside environment is not set up identically to test your code with adaquate coverage. It is then most likely also, in your two previous experiences, that your outside environment was caught lacking in a similar fashion. Often, it is as simple as the fact that there are certain edge cases / specific tests which are not present in the outside environment.

That is now the 3rd KATA, where my Code works in VS, but in CW some tests fail. My code is in c#. With the same numbers, I get differents results in VS and CW!!!!

I like it too

Thank you for the kata 🙏. Not sure if I like it very much though. It checks whether you know particular optimal methods for solving the challenge; solving the challenge is not enough :) Anyway, thanks!

Почему 75 является false, если оно делится на 5, 1, 75, 25 и соответвенно не является простым

I get an error during the full check, even though the numbers that give an error are not prime and are also divisible by another number.

Please show which number failed in the c++ section

Yes I'm in the same boat...

Yes, it is possible. See the "Note on performance" in the description.

The 'attempt' keeps timing out for me.

Not a kata issue. See https://docs.codewars.com/training/troubleshooting#post-discourse and https://docs.codewars.com/training/troubleshooting#timeout.

You do not need sieve for this kata, but the problem with your sieve is that you rebuild it every time for every test (also in a somewhat inefficient way), while it could just be built once and reused between tests.