Ropsten Deployment Error – Fix TypeError: Cannot read property ‘sendTransaction’ of null with hardHat

I am getting this error:

TypeError: Cannot read property 'sendTransaction' of null

when running this command:

$ npx hardhat run scripts/deploy.js --network ropsten

with HardHat on Ropsten network.

This is my deploy.js (which is the one provided by HardHat):

const hre = require("hardhat");

async function main() {

  const Greeter = await hre.ethers.getContractFactory("Greeter");
  const greeter = await Greeter.deploy("Hello, Hardhat!");

  await greeter.deployed();

  console.log("Greeter deployed to:", greeter.address);
}

main()
  .then(() => process.exit(0))
  .catch(error => {
    console.error(error);
    process.exit(1);
  });

This is my App.js:

import { useState } from 'react';
import { ethers } from 'ethers';
import './App.css';
import Greeter from './artifacts/contracts/Greeter.sol/Greeter.json';

function App() {
  const greeterAddress = '0xe7f1725E7734CE288F8367e1Bb143E90bb3F0512';
  const [greeting, SetGreetingValue] = useState('');

  async function requestAccount() {
    await window.ethereum.request({
      method: 'eth_requestAccounts',
    });
  }

  async function fetchGreeting() {
    if (typeof window.ethereum !== 'undefined') {
      const provider = new ethers.providers.Web3Provider(window.ethereum);
      const contract = new ethers.Contract(greeterAddress, Greeter.abi, provider);
      try {
        const data = await contract.greet();
        console.log('data: ', data);
      } catch (error) {
        console.error(error);
      }
    }
  }

  async function setGreeting() {
    if (!greeting) return;
    if (typeof window.ethereum !== 'undefined') {
      await requestAccount();
      const provider = new ethers.providers.Web3Provider(window.ethereum);
      const signer = provider.getSigner();
      const contract = new ethers.Contract(greeterAddress, Greeter.abi, signer);
      const transaction = await contract.setGreeting(greeting);
      SetGreetingValue('')
      await transaction.wait();
      fetchGreeting();
    }
  }

  return (
    <div className="App">
    <button onClick={fetchGreeting}>Fetch Greeting</button>
    <button onClick={setGreeting}>Set Greeting</button>
    <input placeholder='Set greeting' onChange={(e) => SetGreetingValue(e.target.value)} placeholder='Set Greeting' value={greeting}/>
    </div>
  );
}

export default App;

And this is the hardhat.config.js:

module.exports = {
  solidity: "0.8.4",
  paths: {
    artifacts: './client/src/artifacts'
  },
  networks: {
    hardhat: {
      chainId: 1337
    },
    ropsten: {
      url: "https://ropsten.infura.io/v3/938ec750ced64edda8fc17c9e135d0f6",
      account: [`0x08c75f58dcad3bc9664F2ba9Db3C10b2555D6702`]
    }
  }
};

I do not understan why I am getting this sendTransaction error when there is no sendTransaction anywhere in the code