UIToolkit property drawers are only supported when embedded inside a UIToolkit custom editor. This limitation will be addressed in a future release. From the time being, you can declare a custom UIToolkit editor that will look like the default inspector this way:

[CustomEditor(typeof(MyComponent))]
public class MyComponentEditor : Editor
{
public override VisualElement CreateInspectorGUI()
{
var container = new VisualElement();

// If you're running a recent version of the package, or 2021.2, you can use
// InspectorElement.FillDefaultInspector(container, serializedObject, this);
// otherwise, the code below basically does the same thing

SerializedProperty property = serializedObject.GetIterator();
if (property.NextVisible(true)) // Expand first child.
{
do
{
var field = new PropertyField(property);
field.name = "PropertyField:" + property.propertyPath;

if (property.propertyPath == "m_Script")
{
if ((serializedObject.targetObject != null))
field.SetEnabled(false);
}

container.Add(field);
}
while (property.NextVisible(false));
}

return container;
}
}