Username already exist. Why

<form action="" id="manage-user">   
        <input type="hidden" name="id" value="<?php echo isset($meta['id']) ? $meta['id']: '' ?>">
        <div class="form-group">
            <label for="name">Name</label>
            <input type="text" name="name" id="name" class="form-control" value="<?php echo isset($meta['name']) ? $meta['name']: '' ?>" required>
        </div>
        <div class="form-group">
            <label for="username">Username</label>
            <input type="text" name="username" id="username" class="form-control" value="<?php echo isset($meta['username']) ? $meta['username']: '' ?>" required  autocomplete="off">
        </div>
        <div class="form-group">
            <label for="password">Password</label>
            <input type="password" name="password" id="password" class="form-control" value="" autocomplete="off">
            <?php if(isset($meta['id'])): ?>
            <small><i>Leave this blank if you dont want to change the password.</i></small>
        <?php endif; ?>
        </div>
        <?php if(isset($meta['type']) && $meta['type'] == 3): ?>
            <input type="hidden" name="type" value="3">
        <?php else: ?>
        <?php if(!isset($_GET['mtype'])): ?>
        <div class="form-group">
            <label for="type">User Type</label>
            <select name="type" id="type" class="custom-select">
                <option value="2" <?php echo isset($meta['type']) && $meta['type'] == 2 ? 'selected': '' ?>>Staff</option>
                <option value="1" <?php echo isset($meta['type']) && $meta['type'] == 1 ? 'selected': '' ?>>Admin</option>
            </select>
        </div>
        <?php endif; ?>
        <?php endif; ?>
        

    </form>