Неверная обратная связь для выбора опции
<div id="error" class="invalid-feedback d-block">Please make a selection.</div>
Nice Narwhal
<div id="error" class="invalid-feedback d-block">Please make a selection.</div><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>