ドロップダウンリストから対応するオプションが選択された後、それぞれのテーブルに場所が表示された回数を表示します

私が正しく理解していれば、あなたの問題はまさに最後のものと同じだと思います。 WHERE句がありません。

これが機能するかどうかを確認します：

$con = new mysqli("localhost" ,"root" ,"" ,"user_databases");

//query buildings table for the dropdown
$bquery = mysqli_query($con, "SELECT building_ID, building_name FROM buildings");

$selectedbldg = null;

// if the form was submitted
if (!empty($_POST['bldg']))  {
    // store selected building_ID
    $selectedbldg = $_POST['bldg'];
    // the subquery is used to count how many times each location appears
    // for a particular building
    $count = mysqli_query($con, "
        SELECT lo.location_ID, lo.location_name, dt.num_visits
        FROM location lo
        JOIN (
            SELECT location_ID, COUNT(location_ID) AS num_visits
            FROM delivery_transaction 
            WHERE building_ID = {$selectedbldg}
            GROUP BY location_ID
        ) AS dt ON lo.location_ID = dt.location_ID
    ");

    // like before, better to use prepared statement
}
?>

<!-- ... -->

<section class="row text-center placeholders">
    <div class="table-responsive">
        <table class="table table-striped">
            <thead>
                <tr>
                    <th>Location</th>
                    <th>Number of Visits</th>
                </tr>
            </thead>
            <tbody>
            <!-- PHP alternative syntax for control structures: easier to read (imo) -->
            <!-- isset function is to ensure variable $count exist as it only gets declared in the IF condition (you would get an error otherwise) --> 
            <!-- mysqli_num_rows is to check if there are any results to loop over -->
            <?php if (isset($count) && mysqli_num_rows($count)) : ?> 
                <?php while($row = mysqli_fetch_assoc($count)) : ?>
                <tr>
                    <td><?= $row['location_ID'] ?></td>
                    <td><?= $row['num_visits'] ?></td>
                </tr>
                <?php endwhile ?>
            <?php else : ?>
                <tr>
                    <td>No results to display</td>
                </tr>
            <?php endif ?>
            </tbody>
        </table>
    </div>
</section>

読むべきもっと良いもの：