gistfile1.txt

#include <stdio.h>

/*int main(int argc, const char * argv[])*/

int main() {
    
    char str1[256] = "Try to learn";
    char str2[256] = "C and C++ maybe";
    char output[1000];
    
    void concat(char* str1, char* str2, char* output);
    {
        int i, j, k;
        i = j = k = 0;
        
        while (str1[i] != '\0')
        {
            output[k] = str1[i];
            i++;
            k++;
        }
        while (str2 != '\0') 
        {
            output[k] = str2[j];
            k++;
            j++;
        }
        
    }
    
    concat(str1, str2, output);
    printf("%s", output);

}

Move concat() function out of main()
You can't declare a function within a function.