Why am I getting an "Internal Server Error?"

This is what I get in the console:

Repl.it: Updating package configuration

--> /usr/local/bin/python3 -m poetry install

• Serving Flask app "main" (lazy loading)
• Environment: production
  WARNING: This is a development server. Do not use it in a production depl
  oyment.
  Use a production WSGI server instead.
• Debug mode: off * Running on http://0.0.0.0:8080/ (Press CTRL+C to quit)
  GET
  No Post Back Call[2020-04-09 16:25:21,963] ERROR in app: Exception on / [GET]
  Traceback (most recent call last):
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-pac
  kages/flask/app.py", line 2447, in wsgi_app
  response = self.full_dispatch_request()
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-pac
  kages/flask/app.py", line 1952, in full_dispatch_request
  rv = self.handle_user_exception(e)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
  reraise(exc_type, exc_value, tb)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
  raise value
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
  rv = self.dispatch_request()
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
  return self.view_functionsrule.endpoint
  File "main.py", line 22, in index
  return render_template("index.html")
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/templating.py", line 138, in render_template
  ctx.app.jinja_env.get_or_select_template(template_name_or_list),
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/environment.py", line 930, in get_or_select_template
  return self.get_template(template_name_or_list, parent, globals)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/environment.py", line 883, in get_template
  return self._load_template(name, self.make_globals(globals))
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/environment.py", line 857, in _load_template
  template = self.loader.load(self, name, globals)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/loaders.py", line 117, in load
  source, filename, uptodate = self.get_source(environment, name)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/templating.py", line 60, in get_source
  return self._get_source_fast(environment, template)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/templating.py", line 89, in _get_source_fast
  raise TemplateNotFound(template)
  jinja2.exceptions.TemplateNotFound: index.html
  172.18.0.1 - - [09/Apr/2020 16:25:21] "GET / HTTP/1.1" 500 -
  GET
  No Post Back Call
  [2020-04-09 16:25:22,085] ERROR in app: Exception on / [GET]
  Traceback (most recent call last):
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
  response = self.full_dispatch_request()
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
  rv = self.handle_user_exception(e)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
  reraise(exc_type, exc_value, tb)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
  raise value
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
  rv = self.dispatch_request()
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
  return self.view_functionsrule.endpoint
  File "main.py", line 22, in index
  return render_template("index.html")
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/templating.py", line 138, in render_template
  ctx.app.jinja_env.get_or_select_template(template_name_or_list),
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/environment.py", line 930, in get_or_select_template
  return self.get_template(template_name_or_list, parent, globals)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/environment.py", line 883, in get_template
  return self._load_template(name, self.make_globals(globals))
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/environment.py", line 857, in _load_template
  template = self.loader.load(self, name, globals)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/jinja2/loaders.py", line 117, in load
  source, filename, uptodate = self.get_source(environment, name)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/templating.py", line 60, in get_source
  return self._get_source_fast(environment, template)
  File "/home/runner/.local/share/virtualenvs/python3/lib/python3.8/site-packages/flask/templating.py", line 89, in _get_source_fast
  raise TemplateNotFound(template)
  jinja2.exceptions.TemplateNotFound: index.html
  172.18.0.1 - - [09/Apr/2020 16:25:22] "GET / HTTP/1.1" 500 -

I can I avoid this? Thanks!