Users who have not made it past login

On top of what @ireinoso suggested, here are few other ways to approach this:

Create a segment of registered/authenticated users

This type of segment can be helpful to filter other reports or funnels. 

Create a conversion funnel

To directly answer your question of “what percentage of users that visit the site are able to log in”:

Obviously that can be sign up or sign in  depending what you are looking for.

If you are not trying to do an aggregate report over time, do a “conversion rate over time” report:

The power of this type of report is that you can view trends over time.  One of my biggest requests for heap is to combine these 2 report types so we can toggle between aggregate and trending views of these types of reports as they are separate reports now but are really the same report.

Create a custom event

You also said:

“I just want to see how many sessions / users were unable to access the site so that I can recreate an accurate number of logins. “

Given the importance of sign up and sign in on most sites, I recommend if you don’t already have it, to create a custom heap event tied to sign in or sign up completion. This can either be an event created in heap based on a completion success page, or a custom event your engineers implement in your frontend code.  Given importance of this event, I always recommend the latter.  The risk of doing it based on a pageview is that if the URL structures ever change in the future, you’ll be caught chasing URL/pageview definitions for your events (trust me, I speak from experience).