Variable "entry" does not exist This is very basic as I am new to Craft CMS, but I created a Plain Text field in the Homepage section labeled "Tagline". When I enter the Twig code {{ entry.tagline }} into the index.html template, I get the error that the variable "entry" does not exist. 

My index.html is at the root of the public folder.
I would say that the index.html should be within your templates folder. Is it? 
And make sure that you Homepage section is actually pointing to that template. Look at your sections part within Craft CMS.