function buyTokens() public payable {
//CHECK FOR INPUT BALLANCE
require(msg.value > 0, "RGP: INSUFFICIENT INPUT BALLANCE, YOU NEED TO SEND SOME BUSDT");

//CALCULATE THE AMOUNT OF TOKEN TO PURCHASE
uint256 tokensToBuy = mul(msg.value, uint256(10) IERC20(token).decimals()) / mul(price, uint256(10) V2(rgp).decimals());

//ALLOWANCE FOR BOTH ADDRESS(THIS) AND MSG.SENDER
V2(rgp).allowance(msg.sender, address(this));
IERC20(token).allowance(address(this), msg.sender);

//REQUIREMENT FOR ALLOWANCE
tokensToBuy <= V2(rgp).allowance(address(this), msg.sender); // "RGP: CHECK THE TOKEN ALLOWANCE OF ADMIN";
msg.value <= IERC20(token).allowance(msg.sender, address(this)); // "RGP: CHECK THE TOKEN BALANCE OF BUYER";

//APPROVE FOR BOTH MSG.SENDER AND ADDRESS(THIS)
V2(rgp).approve(msg.sender, tokensToBuy);
IERC20(token).approve(address(this), msg.value);

//TRANSFER FROM MSG.SENDER TO ADDRESS(THIS)
IERC20(token).transferFrom(msg.sender, address(this), msg.value);

//CHECK FOR BALANCE OF ADMIN
uint256 rgpBalance = V2(rgp).balanceOf(address(this));

//REQUIRE THAT THE BALANCE APPROVED IS GREATER THAN THE TOKENS TO BUY.
require(rgpBalance >= tokensToBuy, "RGP: NOT ENOUGH TOKEN IN THE RESERVE");

//TRANSFER FROM RGP TO MSG.SENDER
V2(rgp).transferFrom(address(this), msg.sender, tokensToBuy);

//REDUCE THE ALLOWANCE OF BOTH PARTIES
IERC20(token).allowance(msg.sender, address(this)) + msg.value;
V2(rgp).allowance(address(this), msg.sender) + tokensToBuy;

//INCREASE THE ALLOWANCE OF BOTH PARTIES.
IERC20(token).allowance(msg.sender, address(this)) + msg.value;
V2(rgp).allowance(address(this), msg.sender) + tokensToBuy;

//
tokensSold += msg.value;
emit sold(msg.sender, msg.value * price, address(token));

}

function endSale() public onlyOwner{
// Send unsold tokens to the owner.
require(V2(rgp).transfer(owner, address(this).balance), "RGP: Sales ended.");
require(IERC20(token).transfer(owner, address(this).balance), "RGP: Sales ended.");

msg.sender.transfer(address(this).balance);
}
}

please what wrong with it?