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)