Unhandled promise rejection, [TypeError: undefined is not a function]

Question

I have tried almost everything to fix this issue and cannot understand why this is still happening. I'm trying to generate a table and convert to an excel file and the code should be right I just don't understand why this isn't working. This is the file the error is in. If anyone has a clue to why this could be happening please help. The error is

Unhandled promise rejection, [TypeError: undefined is not a function]

import React from 'react';
// Required to save to cache
import * as FileSystem from 'expo-file-system';
// ExcelJS
import ExcelJS from 'exceljs';
// Share excel via share dialog
import * as Sharing from 'expo-sharing';
// From @types/node/buffer
import { Buffer as NodeBuffer } from 'buffer';

import {
  StyleSheet,
  Text,
  View,
  Image,
  SafeAreaView,
  FlatList,
  ActivityIndicator,
  TextInput,
  TouchableHighlight,
  Modal,
  Animated,
  Platform,
  RefreshControl,
  ScrollView,
  Easing,
  Button,
  Dimensions,
  StatusBar
} from "react-native";

import { Sizes, Colors, Padding, Margin, Fonts } from "app/styles"
import GriffinPoolsLogo from "images/griffin-logo.png"
import { addCustomer, getCustomers} from "app/classes/griffin_api.js"
import firebase from 'firebase/compat/app';
import 'firebase/compat/auth';
import 'firebase/compat/firestore';
import Header from 'views/shared/header.js'
import CustomerDetail from 'views/customer_detail.js'
import AddCustomer from 'views/add_customer.js'


const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: 'red',
    height:100,
    width:100,
    alignItems: 'center',
    justifyContent: 'center',
  },
});


// This returns a local uri that can be shared
const generateShareableExcel = async (): Promise<string> => {
  const now = new Date();
  const fileName = 'CustomerData.xlsx';
  const fileUri = FileSystem.cacheDirectory + fileName;
  return new Promise<string>((resolve, reject) => {
    const workbook = new ExcelJS.Workbook();
    workbook.creator = 'Me';
    workbook.created = now;
    workbook.modified = now;
    // Add a sheet to work on
    const worksheet = workbook.addWorksheet('My Sheet', {});
    // Just some columns as used on ExcelJS Readme
    worksheet.columns = [
      { header: 'Id', key: 'id', width: 10 },
      { header: 'Name', key: 'name', width: 32 },
      { header: 'D.O.B.', key: 'dob', width: 10, }
    ];
    // Add some test data
    worksheet.addRow({ id: 1, name: 'John Doe', dob: new Date(1970, 1, 1) });
    worksheet.addRow({ id: 2, name: 'Jane Doe', dob: new Date(1969, 2, 3) });

    // Test styling

    // Style first row
    worksheet.getRow(1).font = {
      name: 'Comic Sans MS', family: 4, size: 16, underline: 'double', bold: true
    };
    // Style second column
    worksheet.eachRow((row, rowNumber) => {
      row.getCell(2).font = {
        name: 'Arial Black',
        color: { argb: 'FF00FF00' },
        family: 2,
        size: 14,
        bold: true
      };
    });

    // Write to file
    workbook.xlsx.writeBuffer().then((buffer: ExcelJS.Buffer) => {
      // Do this to use base64 encoding
      const nodeBuffer = NodeBuffer.from(buffer);
      const bufferStr = nodeBuffer.toString('base64');
      FileSystem.writeAsStringAsync(fileUri, bufferStr, {
        encoding: FileSystem.EncodingType.Base64
      }).then(() => {
        resolve(fileUri);
      });
    });
  });
}


const shareExcel = async () => {
  const shareableExcelUri: string = await generateShareableExcel();
  console.log(shareableExcelUri, "URI")
  Sharing.shareAsync(shareableExcelUri, {
    mimeType: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet', // Android
    dialogTitle: 'Your dialog title here', // Android and Web
    UTI: 'com.microsoft.excel.xlsx' // iOS
  }).catch(error => {
    console.error('Error', error);
  }).then(() => {
    console.log('Return from sharing dialog');
  });
}

export default function GenerateExcel() {
  console.log(generateShareableExcel, "fjkla")
  return(
    <View style={{width:"100%", height:180, marginTop:-Margin.xxlargeMargin * 1.5, backgroundColor:Colors.primaryLight}}>
      <TouchableHighlight
      style={{}}
      onPress={shareExcel}
      >
        <Text style={{color:'white'}}>Generate Table</Text>
      </TouchableHighlight>
    </View>
  );
}