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The first time when i run this it runs fine but the second time when i call the coinFlip() function it shows gas estimation failed can someone tell me why it is happening.

pragma solidity ^0.6.0;

import 'https://github.com/provable-things/ethereum-api/blob/master/provableAPI_0.6.sol';

contract CoinFlipOracle is usingProvable {

string public result;
bytes32 public oraclizeId;

function coinFlip() public payable returns(string memory) {
    oraclizeId = provable_query('WolframAlpha', 'flip a coin');
    return result;
}

function __callback(bytes32 _oraclizeId, string memory _result) public override(usingProvable) {
    // this function should be only call from oracalize service
    require(msg.sender == provable_cbAddress());
    result = _result;
}

}

error message

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You need provide some ether along with the gas fees. Only the first call in provable oracle is free.

edit: provide atleast 0.1 usd worth Ether. (Approx 0.0005)

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