Transfer function doesn't work

Question

I am pretty new at solidity and I am trying to create a roulette where you can place bets on a color or number and then resolve the bets and receive the amount of currency you won. Currently I am doing it on remix with test ether accounts and I am having problems transferring the winnings for a bet from the contract to the current users address. Any ideas? Thank you in advance.

Also I have noticed when trying to return winnings for just a single bet at a time it doesn't work only for the first win when I launch the app and after that the transfer works.

The error I get when I try:

transact to roulette.spinAll errored: VM error: revert. revert The transaction has been reverted to the initial state. Note: The called function should be payable if you send value and the value you send should be less than your current balance. Debug the transaction to get more information.

pragma solidity ^0.5.0;

contract roulette {
    
    
    enum BetType { Color, Number }
    
    struct Bet {
        address user;
        uint amount;
        uint block;
        int choice;
        BetType betType;
    }
    
    uint public counter = 0;
    int public result;
    
    mapping(uint => Bet) public bets;
    mapping(int => int) public colors;
    
    uint public constant NUM_POCKETS = 38;
    
    uint8[18] public RED_NUMBERS = [
        1, 3, 5, 7, 9, 12,
        14, 16, 18, 19, 21, 23,
        25, 27, 30, 32, 34, 36
        ];
        
    uint8[18] public BLACK_NUMBERS = [
        2, 4, 6, 8, 10, 11,
        13, 15, 17, 20, 22, 24,
        26, 28, 29, 31, 33, 35
        ];
        
    function Start() public {
        for(uint i = 0; i < 18; i++) {
            colors[RED_NUMBERS[i]] = 1;
        }

    }
        
    function makeBet(BetType _betType, int _choice) payable public {
        if(_betType == BetType.Color)
            require(_choice == 0 || _choice == 1);
        else require(_choice >= -1 && _choice <=36);
        counter++;
        bets[counter] = Bet(msg.sender, msg.value, block.number, _choice, _betType);
    }
    
    function spinAll() public {
        bytes32 random = keccak256(abi.encodePacked(now, msg.sender));
        result = int(uint(random) % NUM_POCKETS) - 1;
        for(uint i = 0; i < counter; i++) {
            resolveBets(i);
        }
        counter = 0;
    }
    
    function resolveBets(uint id) payable public {
        Bet storage bet = bets[id];
        require(msg.sender == bet.user);
        if(bet.betType == BetType.Color) {
            if(bet.choice == colors[result] && result > 0)
                msg.sender.transfer(bet.amount * 2);       //    < --- This does not work!!!
                
        }
        else if(bet.betType == BetType.Number)
        {
            if(result == bet.choice)
                msg.sender.transfer(bet.amount * 35);
        }
        
        delete bets[id];
    }
}

Answer 1

I think you should try payable(msg.sender).transfer(bet.amount * 35); payable keyword should be mentioned before msg.sender