In Dash Plotly how do I upload CSV with dcc.Upload to store using dcc.Store and then display table?

Your code was missing a few adjustments to run as expected.

First, as sometimes the values can be inside a list you need to get only the value inside the list, in this case, it's in index 0.

content_string = contents[0].split(',')

Second, to avoid some warnings I have added a condition that will not run the element if the upload values are empty.

if [contents, list_of_names, list_of_dates] == [None, None, None]:
    return dash.no_update

You can also use it in your callback declaration:
prevent_initial_call=True

And as a tip, I would suggest you use debug=True n your development environment, so you can have better feedback about what's going wrong, making it easier to debug your code to fix the error

app.run_server(debug=True)

Full code based on your example:

import base64
import datetime
import io

import dash
from dash.dependencies import Input, Output, State
from dash import dcc, html, dash_table

import pandas as pd

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div([
    dcc.Upload(
        id='upload-data',
        children=html.Div([
            'Drag and Drop or ',
            html.A('Select Files')
        ]),
        style={
            'width': '100%',
            'height': '60px',
            'lineHeight': '60px',
            'borderWidth': '1px',
            'borderStyle': 'dashed',
            'borderRadius': '5px',
            'textAlign': 'center',
            'margin': '10px'
        },
        # Allow multiple files to be uploaded
        multiple=True
    ),
    dcc.Store(id='store'),
    html.Div(id='output-data-upload'),
])

import dash
#Goal here is to store the csv from upload into dcc.Store
@app.callback(Output('store', 'data'),
              Input('upload-data', 'contents'),
              State('upload-data', 'filename'),
              State('upload-data', 'last_modified'),
              prevent_initial_call=True)

def update_output(contents, list_of_names, list_of_dates):

    if [contents, list_of_names, list_of_dates] == [None, None, None]:
        return dash.no_update
    
    content_type, content_string = contents[0].split(',')

    decoded = base64.b64decode(content_string)

    df = pd.read_csv(io.StringIO(decoded.decode('utf-8')))

    return df.to_json(date_format='iso', orient='split') #I have to convert to json format to store in dcc.Store


#Goal here is to then extract dataframe from dcc.store by reading json and output a table
@app.callback(
    Output('output-data-upload', 'children'),
    Input('store', 'data'),
              prevent_initial_call=True
)

def output_from_store(stored_data):
    
    df = pd.read_json(stored_data, orient='split')

    return html.Div([


        dash_table.DataTable(
            df.to_dict('records'),
            [{'name': i, 'id': i} for i in df.columns]
        ),

        html.Hr(),  # horizontal line


    ])



if __name__ == '__main__':
    app.run_server(debug=True)

I hope it can be helpful for you.

Regards, Leonardo