В консоле пишет Uncaught SyntaxError: Unexpected end of input
JS
let a = ''; 
let b = ''; 
let sign = '';
let finish = false;
const digit = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.'];
const action = ['-', '+', 'X', '/'];
const out = document.querySelector('.calc-screen p')
function clearAll () {
  a = '';
  b = '';
  sign = '';
  finish = false;
  out.textContent = 0;
}
document.querySelector('.ac').onclick = clearAll;
document.querySelector('.buttons').onclick = (event) => {
     if(!event.target.classList.contains('btn')) return;
     if(event.target.classList.contains('ac')) return;
     out.textContent = '';
     const key = event.target.textContent; 
     if (digit.includes(key)) {
     a+=key;
     console.log(a, b, sign);
   }
}