This error is letting you know that you did not enable "Less Secure Apps" on your mailbox. If you see this error, please go these links to turn them on.
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​Less Secure Apps (Admin Side): https://admin.google.com/ac/security/lsa
​Less Secure Apps (Inbox Side): https://myaccount.google.com/lesssecureapps?pli=1

Please note that, if you believe you are using the correct login and less secure apps is on then try toggling the settings back and forth - this typically resets the settings so please double check everything is on after you toggle it back on.