JSFiddle - React, Tailwind, and code Playground

HTML

<form class="form_hadej" name="form_hadej" action="index.html" method="post">
  <legend><strong><em>Hádej číslo</em></strong></legend>
  <div class="nastav">
    <label for="c1">Vlož rozsah</label>
    <input type="number"onkeyup="aktivuj(this);" onchange="aktivuj(this);" id="c1" class="cislo" name="cislo1" value="">
    <button type="button" id="spust" name="spust" disabled>Spusť</button>
  </div>
  <br>
  <div class="hraj">
    <label for="c1">Hádej</label>
    <input type="number" id="c2" class="cislo" onkeyup="aktivuj(this, 'hadej');" onchange="aktivuj(this, 'hadej');" name="cislo2" value="">
    <button type="button" id="hadej" name="hadej" disabled>Hádej</button>
  </div>
</form>

JavaScript

aktivuj = (element, buttonid) => {
	let val = parseFloat(element.value);
  let btn = buttonid !== undefined ? document.getElementById(buttonid) : element.nextElementSibling;
  btn.disabled = isNaN(val) ? true : false;
}