GMail error

Hi
I wonder if you can pinpoint the error in my Gmail setup – I thought I followed your excellent instructions to the letter! This is the error log from a test:
Versions:
WordPress: 4.9.4
WordPress MS: No
PHP: 5.6.33
WP Mail SMTP: 1.2.5

Params:
Mailer: gmail
Constants: No
Client ID/Secret: Yes
Auth Code: Yes
Access Token: No

Server:
OpenSSL: Yes
PHP.allow_url_fopen: Yes
PHP.stream_socket_client(): Yes
PHP.fsockopen(): Yes
PHP.curl_version(): Yes

Debug:
Error while sending via Gmail mailer: {
“error”: {
“errors”: [
{
“domain”: “global”,
“reason”: “required”,
“message”: “Login Required”,
“locationType”: “header”,
“location”: “Authorization”
}
],
“code”: 401,
“message”: “Login Required”
}
}
Error while sending via Gmail mailer: {
“error”: {
“errors”: [
{
“domain”: “global”,
“reason”: “required”,
“message”: “Login Required”,
“locationType”: “header”,
“location”: “Authorization”
}
],
“code”: 401,
“message”: “Login Required”
}
}
Error while sending via Gmail mailer: {
“error”: {
“errors”: [
{
“domain”: “global”,
“reason”: “required”,
“message”: “Login Required”,
“locationType”: “header”,
“location”: “Authorization”
}
],
“code”: 401,
“message”: “Login Required”
}
}
Error while sending via Gmail mailer: {
“error”: {
“errors”: [
{
“domain”: “global”,
“reason”: “required”,
“message”: “Login Required”,
“locationType”: “header”,
“location”: “Authorization”
}
],
“code”: 401,
“message”: “Login Required”
}
}

Many thanks