1. When in your Gmail account click on the icon with the 9 small squares in the top right
2. Select 'My Account'
3. Click Security on the left side tab.
4. On the corresponding screen if you scroll down, you will see 'Allow less secure apps:'
This may already be ON but in most cases the default setting is set to OFF.
5. Click the toggle to turn this ON
6. Now the 'Allow less secure apps: ON' should be seen