strconv.Itoa() expects a value of type int, so you have to give it that:

log.Println("The amount is: " + strconv.Itoa(int(charge.Amount)))

But know that this may lose precision if int is 32-bit (while uint64 is 64), also sign-ness is different. strconv.FormatUint() would be better as that expects a value of type uint64:

log.Println("The amount is: " + strconv.FormatUint(charge.Amount, 10))

For more options, see this answer: Golang: format a string without printing?

If your purpose is to just print the value, you don't need to convert it, neither to int nor to string, use one of these:

log.Println("The amount is:", charge.Amount)
log.Printf("The amount is: %d\n", charge.Amount)

C++11 standardized the std::to_string function:

#include <string>

int main()
{
  int64_t value = 128;
  std::string asString = std::to_string(value);
  return 0;
}

There are a few ways to do it:

  strtoll(str, NULL, 10);

This is POSIX C99 compliant.

you can also use strtoimax; which has the following prototype:

 strtoimax(const char *str, char **endptr, int base);

This is nice because it will always work with the local intmax_t ... This is C99 and you need to include <inttypes.h>