You forgot to close the request, remember to always return something!

@app.route('/', methods=['GET', 'POST']) 
def main(): 
	if request.method == 'POST':
		email = request.form['email'] 
		print(db.keys)
		db[email] = ""
		return "done!" # <---
	if request.method == 'GET':
		return render_template('index.html')