error message

Everything had been set up just fine. I don’t know what happened but now my emails are not getting through to gmail. I have no idea what this error message means. Do I start from scratch?

EMAIL DELIVERY ERROR: the plugin WP Mail SMTP v2.3.1 logged this error during the last time it tried to send an email:

Mailer: Gmail
{
“error”: {
“code”: 401,
“message”: “Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.”,
“errors”: [
{
“message”: “Login Required.”,
“domain”: “global”,
“reason”: “required”,
“location”: “Authorization”,
“locationType”: “header”
}
],
“status”: “UNAUTHENTICATED”
}
}