This is a test for the question: https://forum.openzeppelin.com/t/can-not-verify-contracts-by-truffle/15872 in the OpenZeppelin Forum

2_deploy_moonbox.js

const MoonBox = artifacts.require("MoonBox");

module.exports = function (deployer) {
  deployer.deploy(MoonBox);
};

MoonBox.sol

pragma solidity ^0.5.0;


import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20Detailed.sol";

contract MoonBox is ERC20, ERC20Detailed {

    constructor() public ERC20Detailed ("MoonBox", "MBX", 18){
        _mint(msg.sender, 1000000 * (10 ** uint256(decimals())));
    }
}

package.json

{
    "name": "openzeppelin_forum_topic_15872",
    "version": "0.0.1",
    "description": "openzeppelin forum topic",
    "scripts": {
        "test": "npx hardhat test"
    },
    "license": "MIT",
    "dependencies": {
        "@openzeppelin/contracts": "^2.5.0",
        "@truffle/hdwallet-provider": "^1.5.0",
        "truffle-plugin-verify": "^0.5.15"
    }
}

result.md

https://kovan.etherscan.io/address/0x1068A008DBb50d1fdd1adeaa20A7852B600d27E7#code

truffle-config.js

/**
 * Use this file to configure your truffle project. It's seeded with some
 * common settings for different networks and features like migrations,
 * compilation and testing. Uncomment the ones you need or modify
 * them to suit your project as necessary.
 *
 * More information about configuration can be found at:
 *
 * trufflesuite.com/docs/advanced/configuration
 *
 * To deploy via Infura you'll need a wallet provider (like @truffle/hdwallet-provider)
 * to sign your transactions before they're sent to a remote public node. Infura accounts
 * are available for free at: infura.io/register.
 *
 * You'll also need a mnemonic - the twelve word phrase the wallet uses to generate
 * public/private key pairs. If you're publishing your code to GitHub make sure you load this
 * phrase from a file you've .gitignored so it doesn't accidentally become public.
 *
 */

const HDWalletProvider = require('@truffle/hdwallet-provider');
//
// const fs = require('fs');
// const mnemonic = fs.readFileSync(".secret").toString().trim();

module.exports = {
  /**
   * Networks define how you connect to your ethereum client and let you set the
   * defaults web3 uses to send transactions. If you don't specify one truffle
   * will spin up a development blockchain for you on port 9545 when you
   * run `develop` or `test`. You can ask a truffle command to use a specific
   * network from the command line, e.g
   *
   * $ truffle test --network <network-name>
   */

  networks: {
    // Useful for testing. The `development` name is special - truffle uses it by default
    // if it's defined here and no other network is specified at the command line.
    // You should run a client (like ganache-cli, geth or parity) in a separate terminal
    // tab if you use this network and you must also set the `host`, `port` and `network_id`
    // options below to some value.
    //
    // development: {
    //  host: "127.0.0.1",     // Localhost (default: none)
    //  port: 8545,            // Standard Ethereum port (default: none)
    //  network_id: "*",       // Any network (default: none)
    // },
    // Another network with more advanced options...
    // advanced: {
    // port: 8777,             // Custom port
    // network_id: 1342,       // Custom network
    // gas: 8500000,           // Gas sent with each transaction (default: ~6700000)
    // gasPrice: 20000000000,  // 20 gwei (in wei) (default: 100 gwei)
    // from: <address>,        // Account to send txs from (default: accounts[0])
    // websocket: true        // Enable EventEmitter interface for web3 (default: false)
    // },
    // Useful for deploying to a public network.
    // NB: It's important to wrap the provider as a function.
    kovan: {
      provider: () => new HDWalletProvider(YOUR_mnemonic, `https://kovan.infura.io/v3/YOUR-PROJECT-ID`),
      network_id: 42,       // Ropsten's id
      gas: 5500000,        // Ropsten has a lower block limit than mainnet
      confirmations: 2,    // # of confs to wait between deployments. (default: 0)
      timeoutBlocks: 200,  // # of blocks before a deployment times out  (minimum/default: 50)
      skipDryRun: true     // Skip dry run before migrations? (default: false for public nets )
    },
    // Useful for private networks
    // private: {
    // provider: () => new HDWalletProvider(mnemonic, `https://network.io`),
    // network_id: 2111,   // This network is yours, in the cloud.
    // production: true    // Treats this network as if it was a public net. (default: false)
    // }
  },

  // Set default mocha options here, use special reporters etc.
  mocha: {
    // timeout: 100000
  },

  // Configure your compilers
  compilers: {
    solc: {
      version: "0.5.17",    // Fetch exact version from solc-bin (default: truffle's version)
      // docker: true,        // Use "0.5.1" you've installed locally with docker (default: false)
      // settings: {          // See the solidity docs for advice about optimization and evmVersion
      //  optimizer: {
      //    enabled: false,
      //    runs: 200
      //  },
      //  evmVersion: "byzantium"
      // }
    }
  },

  plugins: [
    'truffle-plugin-verify'
  ],
  api_keys: {
    etherscan: "YOUR_ETHERSCAN_KEY"
  }

  // Truffle DB is currently disabled by default; to enable it, change enabled:
  // false to enabled: true. The default storage location can also be
  // overridden by specifying the adapter settings, as shown in the commented code below.
  //
  // NOTE: It is not possible to migrate your contracts to truffle DB and you should
  // make a backup of your artifacts to a safe location before enabling this feature.
  //
  // After you backed up your artifacts you can utilize db by running migrate as follows:
  // $ truffle migrate --reset --compile-all
  //
  // db: {
    // enabled: false,
    // host: "127.0.0.1",
    // adapter: {
    //   name: "sqlite",
    //   settings: {
    //     directory: ".db"
    //   }
    // }
  // }
};