HTML

invalid feedback for select option

<div id="error" class="invalid-feedback d-block">Please make a selection.</div>

invalid feedback for select option

<select id="mySelect" class="selectpicker" required>
    <option value="">Select an option</option>
    <option>foo</option>
    <option>bar</option>
</select>
<div id="error" class="invalid-feedback">Please make a selection.</div>