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)

Use the strconv package's Itoa function.

For example:

package main

import (
    "strconv"
    "fmt"
)

func main() {
    t := strconv.Itoa(123)
    fmt.Println(t)
}

Parsing string into int64 example:

// Use the max value for signed 64 integer. http://golang.org/pkg/builtin/#int64
var s string = "9223372036854775807"
i, err := strconv.ParseInt(s, 10, 64)
if err != nil {
    panic(err)
}
fmt.Printf("Hello, %v with type %s!\n", i, reflect.TypeOf(i))

output:

Hello, 9223372036854775807 with type int64!

https://play.golang.org/p/XOKkE6WWer