You MUST turn off 2 step authentication first if it's turn on.

Configure the Motion Setup Page GMAIL Setting Limited remote viewing support may be provided if related to Hik-Connect Cloud. To enable the SMTP function to work with surveillance DVRs, IP cameras, and other devices that can use 3rd party SMTP service, please follow these instructions. Phone: +1-86 (Toll Free) Limited End User Support: (End-user support is intended for product usage-related inquiries such as playback and export.

When setting up email/SMTP details, the recorder requires various fields to be filled - most of them can be found in the help section when you login to your mailbox from your web browser. For CCTV DVRs that allow email alerts to be sent via a third party SMTP server, Gmail is a great service to use, however, you must email the allow less secure apps in your Gmail. GMAIL Settings to work with Hikvision Device To get GMAIL to work with HIKVISION devices you need to enter the details like so.

Tip: Recorder sends email notifications only when motion detection is enabled.

Hikvision email settings for gmail