When I try to deploy contract, the error comes up:

gas required exceeds allowance or always failing transaction

on mainnet.

contract Crowdsale {
  using SafeMath for uint256;

  // The token being sold
  MintableToken public token;

  // start and end timestamps where investments are allowed (both inclusive)
  uint256 public startTime;
  uint256 public endTime;

  // address where funds are collected
  address public wallet;

  // how many token units a buyer gets per wei
  uint256 public rate;

  // amount of raised money in wei
  uint256 public weiRaised;

   * event for token purchase logging
   * @param purchaser who paid for the tokens
   * @param beneficiary who got the tokens
   * @param value weis paid for purchase
   * @param amount amount of tokens purchased
  event TokenPurchase(address indexed purchaser, address indexed beneficiary, uint256 value, uint256 amount);

  function Crowdsale(uint256 _startTime, uint256 _endTime, uint256 _rate, address _wallet, MintableToken _token) public {
    require(_startTime >= now);
    require(_endTime >= _startTime);
    require(_rate > 0);
    require(_wallet != address(0));
    require(_token != address(0));

    startTime = _startTime;
    endTime = _endTime;
    rate = _rate;
    wallet = _wallet;
    token = _token;

  // fallback function can be used to buy tokens
  function () external payable {

  // low level token purchase function
  function buyTokens(address beneficiary) public payable {
    require(beneficiary != address(0));

    uint256 weiAmount = msg.value;

    // calculate token amount to be created
    uint256 tokens = getTokenAmount(weiAmount);

    // update state
    weiRaised = weiRaised.add(weiAmount);

    token.mint(beneficiary, tokens);
    TokenPurchase(msg.sender, beneficiary, weiAmount, tokens);


  // @return true if crowdsale event has ended
  function hasEnded() public view returns (bool) {
    return now > endTime;

  // Override this method to have a way to add business logic to your crowdsale when buying
  function getTokenAmount(uint256 weiAmount) internal view returns(uint256) {
    return weiAmount.mul(rate);

  // send ether to the fund collection wallet
  // override to create custom fund forwarding mechanisms
  function forwardFunds() internal {

  // @return true if the transaction can buy tokens
  function validPurchase() internal view returns (bool) {
    bool withinPeriod = now >= startTime && now <= endTime;
    bool nonZeroPurchase = msg.value != 0;
    return withinPeriod && nonZeroPurchase;


I believe the issue to be the statement:

require(_startTime >= now);

Just for testing purposes try to change now with a specific epoch value, e.g. 1517784279.

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This battery of validity checks requires input parameters, but nothing is said about the deployment methodology.

require(_startTime >= now);
require(_endTime >= _startTime);
require(_rate > 0);
require(_wallet != address(0));
require(_token != address(0));

A series of input arguments is laid out in the constructor function:

function Crowdsale(uint256 _startTime, uint256 _endTime, uint256 _rate, address _wallet, MintableToken _token)

Any of the above requirements will cause an abort if the inputs don't satisfy the required conditions. To further pinpoint the exact cause of the problem, we would have to see the command to deploy so we can investigate each input.

Also worth noting, this is an old dialect of Solidity (and an old question). It's possible the code is compiled with a pre-Byzantium compiler (0.4.21 or lower) in which case it will probably not deploy on a current public blockchain.

Hope it helps.

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