googleapi：错误400：错误的请求，失败前提条件

I'm trying to get message information from the Google API using a service account and a JWT.

I've already enabled Domain wide delegation. I should probably mention that I'm not a GSuite admin, just a user who is trying to get messages from my own Inbox using a Go script.

func main() {
    data, err := ioutil.ReadFile("JWT_Credentials.json")

    conf, err := google.JWTConfigFromJSON(data, gmail.GmailReadonlyScope)
    client := conf.Client(oauth2.NoContext)

    // Create a new gmail service using the client
    gmailService, err := gmail.New(client)

    fullMessage, err := gmailService.Users.Messages.Get("me", message.Id).Format("metadata").Do()
    if err != nil {
         log.Printf("Error: %v", err)
    }
}

and my JWT credentials are as follows:

{
    "type": "service_account",
    "project_id": "PROJECT_ID",
    "private_key_id": "PRIVATE_KEY_ID",
    "private_key": "-----BEGIN PRIVATE KEY-----\PRIVATE_KEY",
    "client_email": "SERVICE@PROJECT_ID.iam.gserviceaccount.com",
    "client_id": "CLIENT_ID",
    "auth_uri": "https://accounts.google.com/o/oauth2/auth",
    "token_uri": "https://oauth2.googleapis.com/token",
    "auth_provider_x509_cert_url": 
    "https://www.googleapis.com/oauth2/v1/certs",
    "client_x509_cert_url": "CERT_URL"
}

I get the following error when the code runs:

 googleapi: Error 400: Bad Request, failedPrecondition

If anybody has the solution to this problem or has encountered it in the past, please help me out!