Header `execution` and `std::reduce` not found

std::reduce and std::execution::par are available since C++17.

For most of the compilers C++17 isn't fully implemented yet. You can try using clang with flag -std=c++1z.

I upgraded my GCC to version 10 and it compiled fine the std::execution::par with its include <execution> (which was failing to locate with gcc version 7.5.0). I followed to the instructions in this link: https://tuxamito.com/wiki/index.php/Installing_newer_GCC_versions_in_Ubuntu