Hello,

Avec être (to be) or vivre (to live), you CAN use either dans la/le OR nothing at all.
Ce magasin est dans l'avenue Jeanne d'Arc.
This shop is on Avenue Jeanne d'Arc.
Ce magasin est avenue Jeanne d'Arc.
This shop is on Avenue Jeanne d'Arc.
I do NOT take the explanation to mean «with vivre or être we are to use dans or nothing.» It states that you can use either «dans la/le» or nothing. The examples above illustrate this quite nicely; however, the use of «en» differs somewhat in the locution.
Here are a few other links that explain «dans vs en» usage a little differently.
http://www4.ncsu.edu/~dsbeckma/110Dans-en.html
https://www.lawlessfrench.com/grammar/prepositions-of-place/
https://french.stackexchange.com/questions/21/when-to-use-en-vs-dans
The better way to indicate the difference in usage is by a couple of examples, so:
Je suis dans la voiture ---> I am in the car
Je suis en voiture --> I'm in the car
«Does that mean we cannot use en with the verb être.» with your question here, yes en can be used with être as can be seen in the examples.
I get the sense that the confusion stems from various locutions. With continuing to study French, this will become clear with time.
Bonne journée.

Merci beaucoup Ron

You can say "Je suis dans la voiture" and also "je suis en voiture". Both mean different things which can be a bit troublesome to tell apart in English. I'll give it a try: Je suis dans la voiture -- I am in the car. You emphasize that you are inside the car. Je suis en voiture -- You are in the car also but the emphasis is that you travel by car not that you are physicall inside the car (which, of course, you have to be anyway). This distinction between "dans" inside and "en" a more general or inferred inside can also be seen in this example: Je suis dans la classe -- I am in the classroom. Je suis en classe -- I am attending a class (and, of course, I am also in the classroom). Je suis dans la ville -- I am in the town. Je suis en ville -- I am in town. -- Chris (not a native speaker).