未捕获（按承诺）TypeError：无法获取

Question

**尝试从数据库取回数据时出现问题。 请如何解决此错误？ **

    const currentEl_one = document.getElementById('currency-one'); 
    const amountEl_one = document.getElementById('amount-one'); 
    const currentEl_two = document.getElementById('currency-two'); 
    const amountEl_two = document.getElementById('amount-two');  


const rateEl = document.getElementById('rate');  

//获取汇率并更新DOM

const calculate() => {
    const currency_one = currentEl_one.value; 
    const currency_two = currentEl_two.value; 

    fetch(`https://api.exchangerate-api.com/v6/latest/${currency_one}`) 
    .then(res => res.json())
    .then(data => {
        const rate = data.rates[currency_two]; 
        rateEl.innerHTML = `1 ${currency_one} = ${rate} ${currency_two}`; 
        amountEl_two.value = (amountEl_one.value * rate).toFixed(2);
    });
} 

//事件监听器

currentEl_one.addEventListener('change', calculate); 
amountEl_one.addEventListener('input', calculate); 
currentEl_two.addEventListener('change', calculate); 
amountEl_two.addEventListener('input', calculate);  

swap.addEventListener('click', () => {
    const temp = currentEl_one.value; 
    currentEl_one.value = currentEl_two.value; 
    currentEl_two.value = temp; 
    calculate();
})
calculate();