Untitled

<form id="meyveForm" action="#" method="post">
    <p>En az bir meyve seçiniz:</p>
    <input type="checkbox" id="elma" name="meyveler[]" value="elma">
    <label for="elma">Elma</label><br>
    <input type="checkbox" id="armut" name="meyveler[]" value="armut">
    <label for="armut">Armut</label><br>
    <input type="checkbox" id="çilek" name="meyveler[]" value="çilek">
    <label for="çilek">Çilek</label><br><br>
    <input type="submit" value="Gönder">
</form>

<script>
    document.getElementById("meyveForm").addEventListener("submit", function(event) {
        var checkboxes = document.querySelectorAll('input[type="checkbox"]:checked');
        if (checkboxes.length < 1) {
            alert("En az bir meyve seçmelisiniz.");
            event.preventDefault();
        }
    });
</script>