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I’m starting with Flask and I’m using Bootstrap 3.3.7, so I took the example navbar code on their website, but when I start the index Nav bar does not work: 
and the button looks like bootstrap 4, but I’ve downloaded bootstrap 3.3.7 several times and continues the same way.
Screenshots of index.html:
Try this young : <link rel="stylesheet" href="{{ url_for('Static', filename='css/bootstrap.css') }}"> You can open the browser development tools and go to the 'Resources' tab to check whether or not the file has been imported
– Bruno
You haven’t changed a bit. but you’re making this mistake:
Uncaught Error: Bootstrap tooltips require Tether (http://tether.io/)but I never needed anything else to make bootstrap work before.– Vinicius Barbosa de Medeiros
instead of using local files, I used Cdn and it worked so I went to search for this error and saw that it is related to bootstrap 4, as if the files I downloaded were from 4, but I went to the site and downloaded the 3.3.7 so it will not go.
– Vinicius Barbosa de Medeiros