I can't reproduce this. The following program works fine, both for user IDs which are present and ones which are not:

package main

import (
	"fmt"
	"os/user"
)

func main() {
	uu, err := user.LookupId("501")
	if err != nil {
		println("error", err)
		return
	}
	fmt.Printf("%v\n", uu)
}

If you can, please post an entire program, not just a snippet (I'm guessing the user package is os/user in the stdlib, but without a whole program it's hard to be sure).

What Darwin version are you using? I tested on Catalina (10.15.2).

sorry. I found it happens with the cross-complie

1. used cross-complie on ubuntu 18.04 to generate darwin and linux binary
2. run the darwin binary on mac

There is a difference in how the os/user package works by default for a native compile and a cross-compile. By default for a native compile the os/user package uses the C function getpwuid_r. By default for a cross-compile the os/user package reads the /etc/passwd file. This difference in implementation is presumably what is leading to the different behavior.

You can get the native compile behavior when cross-compiling by setting the environment variable CGO_ENABLED=1. But that will require you to have a complete macOS cross-compiler with macOS libraries.