JSFiddle - React, Tailwind, and code Playground

HTML

<input id="b">
<input id="p">
<input type=button value="click me" onclick="CheckInput()">
<div id="error">Hey sorry wrong input</div>

JavaScript

function CheckInput() {
	const billInput = document.getElementById("b").value;
  const peopleAmount = document.getElementById("p").value;

	if ((isNaN(billInput)) || (isNaN(peopleAmount)) || (billInput === "") || (peopleAmount === "")) {
  	showErr();
  }
  else{
  	hideErr();
  }
}

function hideErr(){
	console.log("hide");
	const el = document.getElementById("error");
  el.style.display = "none";
}
function showErr(){
	console.log("show");
	const el = document.getElementById("error");
  el.style.display = "block";
  el.innerHTML = "Hey sorry wrong input";
}

window.onload = function() {
	hideErr();
}