Transfer ERC20 DAI Tokens to Contract

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

interface IERC20 {
function totalSupply() external view returns (uint256);

function balanceOf(address account) external view returns (uint256);

function transfer(address recipient, uint256 amount)
    returns (bool);

function allowance(address owner, address spender)
    returns (uint256);

function approve(address spender, uint256 amount) external returns (bool);

function transferFrom(
    address sender,
    address recipient,
    uint256 amount
) external returns (bool);

event Transfer(address indexed from, address indexed to, uint256 value);
event Approval(
    address indexed owner,
    address indexed spender,
    uint256 value


contract contractB {
address tracker_0x_address = 0xaD6D458402F60fD3Bd25163575031ACDce07538D; // Ropsten DAI Address
mapping(address => uint256) public balances;

function deposit(uint256 tokens) public {
    // add the deposited tokens into existing balance
    balances[msg.sender] += tokens;

    // transfer the tokens from the sender to this contract
    IERC20(tracker_0x_address).approve(msg.sender, tokens);

function balance() public view returns (uint256) {
    return IERC20(tracker_0x_address).balanceOf(msg.sender);

function returnTokens() public {
    balances[msg.sender] = 0;
    IERC20(tracker_0x_address).transfer(msg.sender, balances[msg.sender]);


This is my code and I am not able to transfer DAI to my contract.
I am not able to approve smart contract to spend our DAI

Please help
Thanks in Advance