...
When you connect your Gmail account, additional steps are needed for you to connect successfully.
...
When you first attempt to connect you will most likely get the following error message but continue reading and follow the steps on how to enable POP and allow Less Secure Apps.
How to enable POP Access in Gmail
1. Click the Gear icon in the top right corner of the screen and click See all Settings.
...
2. Click the Forwarding and POP/IMAP tab on the settings screen and under POP Download click Enable POP for all mail and click Save Changes.
...
How to allow Less Secure Apps in Gmail
You will also need to enable less secure apps by going to your Gmail and click a link from there. Alternatively you can use the following link to enable less secure apps.
...