Flask raises TemplateNotFound error even though template file exists

You must create your template files in the correct location; in the templates subdirectory next to the python module (== the module where you create your Flask app).

The error indicates that there is no home.html file in the templates/ directory. Make sure you created that directory in the same directory as your python module, and that you did in fact put a home.html file in that subdirectory. If your app is a package, the templates folder should be created inside the package.

myproject/
    app.py
    templates/
        home.html

myproject/
    mypackage/
        __init__.py
        templates/
            home.html

Alternatively, if you named your templates folder something other than templates and don't want to rename it to the default, you can tell Flask to use that other directory.

app = Flask(__name__, template_folder='template')  # still relative to module

You can ask Flask to explain how it tried to find a given template, by setting the EXPLAIN_TEMPLATE_LOADING option to True. For every template loaded, you'll get a report logged to the Flask app.logger, at level INFO.

This is what it looks like when a search is successful; in this example the foo/bar.html template extends the base.html template, so there are two searches:

[2019-06-15 16:03:39,197] INFO in debughelpers: Locating template "foo/bar.html":
    1: trying loader of application "flaskpackagename"
       class: jinja2.loaders.FileSystemLoader
       encoding: 'utf-8'
       followlinks: False
       searchpath:
         - /.../project/flaskpackagename/templates
       -> found ('/.../project/flaskpackagename/templates/foo/bar.html')
[2019-06-15 16:03:39,203] INFO in debughelpers: Locating template "base.html":
    1: trying loader of application "flaskpackagename"
       class: jinja2.loaders.FileSystemLoader
       encoding: 'utf-8'
       followlinks: False
       searchpath:
         - /.../project/flaskpackagename/templates
       -> found ('/.../project/flaskpackagename/templates/base.html')

Blueprints can register their own template directories too, but this is not a requirement if you are using blueprints to make it easier to split a larger project across logical units. The main Flask app template directory is always searched first even when using additional paths per blueprint.

I think Flask uses the directory template by default. So your code should be like this

suppose this is your hello.py

from flask import Flask,render_template

app=Flask(__name__,template_folder='template')


@app.route("/")
def home():
    return render_template('home.html')

@app.route("/about/")
def about():
    return render_template('about.html')

if __name__=="__main__":
    app.run(debug=True)

And you work space structure like

project/
    hello.py        
    template/
         home.html
         about.html    
    static/
           js/
             main.js
           css/
               main.css

also you have create two html files with name of home.html and about.html and put those files in templates folder.

I don't know why, but I had to use the following folder structure instead. I put "templates" one level up.

project/
    app/
        hello.py
        static/
            main.css
    templates/
        home.html
    venv/

This probably indicates a misconfiguration elsewhere, but I couldn't figure out what that was and this worked.

(Please note that the above accepted Answer provided for file/project structure is absolutely correct.)

Also..

In addition to properly setting up the project file structure, we have to tell flask to look in the appropriate level of the directory hierarchy.

for example..

    app = Flask(__name__, template_folder='../templates')

    app = Flask(__name__, template_folder='../templates', static_folder='../static')

Starting with ../ moves one directory backwards and starts there.

Starting with ../../ moves two directories backwards and starts there (and so on...).

Hope this helps