Я думаю, что ваша структура вывода неверна, вы могли бы вернуть html
намного лучше, но я даю свое решение на основе вашего текущего вывода:
$('.Selects').change(function() {
$(this).prev('label').children('input[type="checkbox"]').attr('checked', true)
});
$('.Selects').change(function() {
$(this).prev('label').children('input[type="checkbox"]').attr('checked', true)
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<label><input type='checkbox' name='location[]' id='location_FF' class="CheckBoxes" value='FF' /> Front Flat </label>
<select name='addWhat[FF]' id='addWhat_FF' class="Selects">
<option value=''> -</option>
<option value='Logo'>Logo</option>
<option value='Nam'>Name</option>
<option value='Num'>Number</option>
<option value='Mon'>Monogram</option>
</select>
<label><input type='checkbox' name='location[]' id='location_FF' class="CheckBoxes" value='FF' /> Front Flat </label>
<select name='addWhat[FF]' id='addWhat_FF' class="Selects">
<option value=''> -</option>
<option value='Logo'>Logo</option>
<option value='Nam'>Name</option>
<option value='Num'>Number</option>
<option value='Mon'>Monogram</option>
</select>
<label><input type='checkbox' name='location[]' id='location_FF' class="CheckBoxes" value='FF' /> Front Flat </label>
<select name='addWhat[FF]' id='addWhat_FF' class="Selects">
<option value=''> -</option>
<option value='Logo'>Logo</option>
<option value='Nam'>Name</option>
<option value='Num'>Number</option>
<option value='Mon'>Monogram</option>
</select>
И учтите, что не используйте дубликаты ID
!
location_FF
Улучшенная версия:
$('.Selective select').change(function() {
$(this).parent().find('input[type="checkbox"]').attr('checked', true)
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="Selective">
<label><input type='checkbox' name='location[]' value='FF' /> Front Flat </label>
<select name='addWhat[FF]'>
<option value=''> -</option>
<option value='Logo'>Logo</option>
<option value='Nam'>Name</option>
<option value='Num'>Number</option>
<option value='Mon'>Monogram</option>
</select>
</div>
<div class="Selective">
<label><input type='checkbox' name='location[]' value='FF' /> Front Flat </label>
<select name='addWhat[FF]'>
<option value=''> -</option>
<option value='Logo'>Logo</option>
<option value='Nam'>Name</option>
<option value='Num'>Number</option>
<option value='Mon'>Monogram</option>
</select>
</div>
<div class="Selective">
<label><input type='checkbox' name='location[]' value='FF' /> Front Flat </label>
<select name='addWhat[FF]'>
<option value=''> -</option>
<option value='Logo'>Logo</option>
<option value='Nam'>Name</option>
<option value='Num'>Number</option>
<option value='Mon'>Monogram</option>
</select>
</div>