Solidity Type Conversion – Handling Explicit Type Conversion in IERC20 Contracts

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}      

I changed your smart contract implementation with this smart contract.

I put notes just so you'd know your errors.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// NOTE: Import IERC20 contract
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

// NOTE: You must to implement IERC20 for override ERC20 methods!
contract NewToken is IERC20 {

  string public constant name = "NewToken";
  string public constant symbol = "NTKN";
  uint8 public constant decimals = 18;
  uint256 totalSupply_ = 10000000000000000000000000; 

  mapping(address => uint256) public balances;
  mapping(address => mapping (address => uint256)) allowed;

  // NOTE: I removed the events because it already defined into IERC20
  
  constructor() { 
    balances[msg.sender] = totalSupply_;
  }

  function totalSupply() public override view returns (uint256) {
    return totalSupply_;
  }

  // NOTE: You must to change signature function adding address parameter
  function balanceOf(address _address) public override view returns(uint256) {
    return balances[_address];
  }

  function transfer(address to, uint256 value) public override returns (bool) {
      require(value <= balances[msg.sender]);
      balances[msg.sender] = balances[msg.sender]-value;
      balances[to] = balances[to]+value;
      // NOTE: The event is called  Transfer not 'transfer'
      emit Transfer(msg.sender, to, value);
      return true;
  }

  function approve(address spender, uint256 value) public override returns (bool) {
      allowed[msg.sender][spender] = value;
      emit Approval(msg.sender, spender, value);
      return true;
  }

  function allowance(address owner, address spender) public override view returns (uint) {
      return allowed[owner][spender];
  }

  function transferFrom(address from, address to, uint256 value) public override returns (bool) {
      require(value <= balances[from]);
      require(value <= allowed[from][msg.sender]);

      balances[from] = balances[from]-value;
      allowed[from][msg.sender] = allowed[from][msg.sender]+value;
      balances[to] = balances[to]+value;
      emit Transfer(from, to, value);
      return true;
  }
}