This is the only solution that passed all test. Alhamdulillah! I am proud of this one.

currency-conv.js

let input = require('sync-input');

console.log("Welcome to Currency Converter!");
console.log(`1 USD equals  1 USD
1 USD equals  113.5 JPY
1 USD equals  0.89 EUR
1 USD equals  74.36 RUB
1 USD equals  0.75 GBP`);
console.log("I can convert USD to these currencies: JPY, EUR, RUB, USD, GBP");
console.log("Type the currency you wish to convert: USD");

let currency = input("To:");
(function checkCurrency() {
  if(currency == "JPY" || currency == "jpy" || currency == "JpY") {
    // do something
    convertCurrency(currency, 113.5);
  } else if(currency == "EUR" || currency == "eur" || currency == "EuR") {
    // do something
    convertCurrency(currency, 0.89);
  } else if(currency == "RUB" || currency == "rub" || currency == "RuB") {
    // do something
    convertCurrency(currency, 74.36);
  } else if(currency == "GBP" || currency == "gbp" || currency == "GbP") {
    // do something
    convertCurrency(currency, 0.75);
  } else if(currency == "USD" || currency == "usd" || currency == "UsD") {
    // do something
    convertCurrency(currency, 1);
  } else {
    // unknown currency
    console.log("Unknown currency");
  }
}());

function convertCurrency(curr, val) {
  let amount = input("Amount:");
  let result;
  
  // checks on amount
  if(amount < 1) {
    result = "The amount can not be less than 1";
  } else if(isNaN(amount)) {
    result = "The amount has to be a number";
  } else {
    result = amount * val;
  }

  // show result
  isNaN(result) ? console.log(result) : console.log("Result: " + amount + " USD equals " + parseFloat(result).toFixed(4) + " " + curr.toUpperCase());
}