Server Error 403 Forbidden

I have followed this guide:
https://wpmailsmtp.com/docs/how-to-set-up-the-gmail-mailer-in-wp-mail-smtp/

but when I try the last step “3. Grant Permissions and Send Test Email”
and click the button “Allow plugin to send emails using your Google account”
and then click the Google button “Allow”
it should send me back to my WP-admin page, but I get an error “Server Error 403 Forbidden. You do not have permission to access this document.“