Callback warning: Callback error creating dash' DataTable

The reason it is not working is because you're not handling the case in your callback where the button has not been clicked. Even if the button hasn't been clicked your callback still needs to return a tuple (of your data and columns).

That is what the error is saying here:

Expected the output type to be a list or tuple but got: None.

You don't return anything (None) when n_clicks is 0 (false).

So what you need to do is return the data and columns when n_clicks is 0. The only difference in approach from what you have is that you don't apply any filters to the data if n_clicks is 0.

Full example:

import dash
import dash_html_components as html
import dash_core_components as dcc
import dash_table as dt
from dash.dependencies import Input, Output, State
from dash.exceptions import PreventUpdate
import pandas as pd

df = pd.read_csv("https://raw.githubusercontent.com/plotly/datasets/master/solar.csv")

app = dash.Dash(__name__)

states = df.State.unique().tolist()
numSP = df["Number of Solar Plants"].unique().tolist()

app.layout = html.Div(
    [
        dcc.Dropdown(
            id="filter_dropdown",
            options=[{"label": st, "value": st} for st in states],
            value=states[0],
        ),
        html.Br(),
        dcc.Dropdown(
            id="filter2",
            options=[{"label": np, "value": np} for np in numSP]
            # value = numSP[0]
        ),
        html.Br(),
        html.Button("Submit", id="submit-button-state", n_clicks=0),
        html.Br(),
        html.Div(id="output"),
        html.Div(
            # id ='priceTable'
            dt.DataTable(id="priceTable")
        ),
    ]
)


def get_table_data(data):
    columns = [
        {
            "name": i,
            "id": i,
        }
        for i in (data.columns)
    ]
    data = data.to_dict("records")
    return (data, columns)


@app.callback(
    [Output("priceTable", "data"), Output("priceTable", "columns")],
    [Input("submit-button-state", "n_clicks")],
    [State("filter_dropdown", "value")],
    [State("filter2", "value")],
    [State("submit-button-state", "n_clicks")],
)
def update_table(n_clicks, filter_dropdown, filter2, table):
    if n_clicks:
        if filter_dropdown is None and filter2 is None:
            raise PreventUpdate
        if filter_dropdown is not None and filter2 is not None:
            table = df[df.State == filter_dropdown]
            table = table[table["Number of Solar Plants"] == filter2]
        if filter_dropdown is None and filter2 is not None:
            table = df[df["Number of Solar Plants"] == filter2]
        if filter_dropdown is not None and filter2 is None:
            table = df[df.State == filter_dropdown]

        (data, columns) = get_table_data(table)
        print("columns", columns)
        print("data", columns)

        return data, columns

    return get_table_data(df)


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

In the example above I've also put your code for retrieving data and columns from a dataframe in its own function get_table_data. For making sure a tuple of your data is always returned I've used a guard clause.