It's great, but it logs errors because there's no elements with the ids nextCost1 or nextCost2.
here I've commented out the bad lines

function BuySeniorHacker() {
  var SeniorCost = Math.floor(100 * Math.pow(1.2, Senior));
  if (Hacked >= SeniorCost) {
    Senior = Senior + 1;
    Hacked = Hacked - SeniorCost;
    HackPerClick = HackPerClick + 5;
    document.getElementById("Senior").innerHTML = Senior;
    document.getElementById("InfoHacked").innerHTML = Hacked;
    localStorage.setItem("Hacked", Hacked);
    localStorage.setItem("HackPerClick", HackPerClick);
    localStorage.setItem("Senior", Senior);
  }
  var nextCost1 = Math.floor(100 * Math.pow(1.2, Senior));
  document.getElementById("SeniorCost").innerHTML = nextCost1;
  //document.getElementById("nextCost1").innerHTML = SeniorCost;
  localStorage.setItem("nextCost1", nextCost1);
  localStorage.setItem("SeniorCost", SeniorCost);
}
function BuyProHacker() {
  var ProCost = Math.floor(1000 * Math.pow(1.2, Pro));
  if (Hacked >= ProCost) {
    Pro = Pro + 1;
    cps = cps + 5;
    HackPerClick = HackPerClick + 10;
    Hacked = Hacked - ProCost;
    document.getElementById("Pro").innerHTML = Pro;
    document.getElementById("InfoHacked").innerHTML = Hacked;
    localStorage.setItem("Hacked", Hacked);
    localStorage.setItem("Pro", Pro);
    localStorage.setItem("HackPerClick", HackPerClick);
    localStorage.setItem("cps", cps);
  }
  var nextCost2 = Math.floor(1000 * Math.pow(1.2, Pro));
  document.getElementById("ProCost").innerHTML = nextCost2;
  //document.getElementById("nextCost2").innerHTML = ProCost;
  localStorage.setItem("ProCost", ProCost);
  localStorage.setItem("nextCost2", nextCost2)
}