Note This answer only applies to version 2.x of Vue. Version 3 has lifted this restriction.
You have two root elements in your template.
<div class="form-group">
...
</div>
<div class="col-md-6">
...
</div>
And you need one.
<div>
<div class="form-group">
...
</div>
<div class="col-md-6">
...
</div>
</div>
Essentially in Vue you must have only one root element in your templates.
The best answer