Cannot submit transactions due to "it seems this transaction will fail"

Question

I am following a tutorial to write a registrant contract and when I try to submit transactions to the contract I always get error :it seems this transaction will fail. If you submit it, it may consume all the gas upi provide.
I create run a private block chain on my laptop and I deployed the contract through Mist. The source code are shown below:

pragma solidity ^0.4.18;

contract EventRegistrant {

    //basic class of registrant
    struct Registrant {
        uint amount;
        uint numTickets;
        string email;
    }

    //define variables
    address public owner;
    uint public numTicketsSold;
    uint public quota;
    uint public price;
    mapping (address => Registrant) registrantsPaid;
    
    event Deposit (address _from, uint _amount);
    event Refund (address _to, uint _amount);

    //Modifiers to define following functions
    //character "_" means this is a modifier
    modifier onlyOwner(){
        require(msg.sender != owner);
        _;
    }
    
    modifier soldOut(){
        require(numTicketsSold >= quota);
        _;
    }

    function EventRegistrant(uint _quota, uint _price) public {
        owner = msg.sender;
        numTicketsSold = 0;
        quota = _quota;
        price = _price;
    }

    function buyTicket(string email, uint numTickets) soldOut public payable {
        uint totalAmount = price * numTickets;
        require(msg.value > totalAmount);
        if (registrantsPaid[msg.sender].amount > 0) {
            registrantsPaid[msg.sender].amount += msg.value;
            registrantsPaid[msg.sender].numTickets += numTickets;
            registrantsPaid[msg.sender].email = email;
        } else {
            registrantsPaid[msg.sender].amount = totalAmount;
            registrantsPaid[msg.sender].numTickets = numTickets;
            registrantsPaid[msg.sender].email = email;
        }
        
        numTicketsSold += numTickets;
        
        if (msg.value > totalAmount) {
            uint refundAmount = msg.value - totalAmount;
            require(msg.sender.send(refundAmount));
        }
        
        Deposit(msg.sender, msg.value);
    }
    
    function refundTickets(address buyer) onlyOwner public {
        if (registrantsPaid[buyer].amount > 0) {
            if (this.balance >= registrantsPaid[buyer].amount) {
                registrantsPaid[buyer].amount = 0;
                numTicketsSold = numTicketsSold - registrantsPaid[buyer].numTickets;
                require(buyer.send(registrantsPaid[buyer].amount));
                Refund(buyer, registrantsPaid[buyer].amount);
            }
        }
    }
    
    function withdrawFunds() onlyOwner public {
        require(owner.send(this.balance));
    }

    function getRegistrantsAmountPaid(address buyer) public view returns (uint) {
        return registrantsPaid[buyer].amount;
    }

    //destructor, to delete the contract when there is no need
    function kill() onlyOwner public {
        selfdestruct(owner);
    }
}

This is how my contract looks like in the Mist. This is error when I submit the transaction.

Anyone can help me? thank you in advance.

Answer 1

buyTickets seems to only allow someone to buy a ticket if all the tickets are already sold (soldOut). Even if this isn't the source of the problem, it's almost certainly a mistake.