Cannot POST simple HTML form with Flask (Python)

  flask, html, python

This is an incredibly simple question and I am ashamed to be asking it here, but I have been Googling answers for the past 3 hours with no results.

Basically, I am trying to send information using an HTML form and accept that information with a Flask python script. The problem is whenever I try to submit the form, it says Cannot POST path/form.html

The HTML Form:

<form method="post"> 
<label for="firstname">First Name:</label>
<input type="text" id="firstname" name="fname" placeholder="firstname">
<label for="lastname">Last Name:</label>
<input type="text" id="lastname" name="lname" placeholder="lastname">
<button type="submit">Login</button>

The Python/Flask Code:

from flask import Flask, request, render_template  

# Flask constructor
app = Flask(__name__)

# A decorator used to tell the application
# which URL is associated function
@app.route('/', methods =["GET", "POST"])
def gfg():
    if request.method == "POST":
       # getting input with name = fname in HTML form
       first_name = request.form.get("fname")
       # getting input with name = lname in HTML form
       last_name = request.form.get("lname")
       return "Your name is "+first_name + last_name
    return render_template("form.html")

if __name__=='__main__':

I know this should be simple but web development is not my forte and this is really out of my element. Is this a server problem? Thank you in advance.

Source: Python Questions