It looks like you want to compare strings after sorting. Assuming your sort function does the right thing, you need to compare the strings after sorting them.
sort(string1);
sort(string2);
val = strcmp(string1, string2);
The reason for the error is that your sort function returns void. So you are effectively passing void arguments to strcmp. And that can’t work.
The way to do this in C++ would be to use std::string, and call std::sort.
std::string string1, string2;
std::cout << "Enter first string";
std::cin >> string1;
std::cout << "Enter second string";
std::cin >> string2;
std::sort(string1.begin(), string1.end());
std::sort(string2.begin(), string2.end());
bool val = string1 == string2;
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solved Invalid use of void expression in strcmp