The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again

I don't know if it's due to some other python application running or something, but when I was a beginner in Flask I got a similar error to you.

I wrote app.route('/') instead of @app.route('/') (I missed the @ symbol).

See whether it's the case for your code.

You should have written @app.route('/home/'). That fixed the problem for me!

My solution to this problem: is to switch this instruction

if __name__ == "__main__":
     app.run(debug=True ,port=8080,use_reloader=False)

as in the example bellow

from flask import Flask, render_template, request                     
app = Flask(__name__,template_folder="template")          
                                                          
@app.route("/")
def hello():
    return render_template('index.html') 

if __name__ == "__main__":
     app.run(debug=True ,port=8080,use_reloader=False) 
@app.route('/register', methods=['POST'])
def register():
   query = request.form.get('query1')
   selected = request.form.get('query')
   if (not query or not selected):
       return 'failure'
   processed_text = query.upper()
   return render_template('sucess.html')

to the end of your script (same example):

 from flask import Flask, render_template, request
 app = Flask(__name__,template_folder="template")

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


 @app.route('/register', methods=['POST'])
 def register():
     query = request.form.get('query1')
     selected = request.form.get('query')
     if (not query or not selected):
        return 'failure'
     processed_text = query.upper()
     return render_template('sucess.html')

 if __name__ == "__main__":
     app.run(debug=True ,port=8080,use_reloader=False)

UPDATE---------------------------------

You should also take care of the local file. Please put them in the statistic field then use url_for from the Flask framework to create a correct URL

<div style = "text-align: center;">
  <a href="{{ url_for('static',filename=path)}}">
    <button class="slide_from_bottom button">check here </button>
  </a>
</div>

I initially tried with @app.route("/hello") where the function I defined inside was def hello(): return "some statement" this gave me the error stated above, I removed "/hello" and tried with "/" this resolved the issue for me. p.s(I did this in pycharm and os is windows 10)

I had this issue after deploying a legacy Python Flask application to Azure App Service for my team.

I was getting the error:

Not Found. The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.

Here's how I fixed it:

The issue was that the URL I was entering wasn't the right one. Apparently, the developers did not set a root route, so instead of this:

https://my-website.com

You will have to specify the path to the resource you want, so we will have:

https://my-website.com/my-desired-path

That's all

You might have missed @ before the app.route('/')