A family of Microsoft relational database management systems designed for ease of use.
There are very few options for how a query's datasheet should look and Nav buttons is not one of those few. Use a real form as a subform (with its record source set to the query) and set the form's NavigationButtons property to No.
The NavigationButtons property is a property of a form object, it is not a property of a subform control that is used to display a query's datasheet or a form object.
It sounds like you clicked once on the subform control in the main form's design view. If you click a second time, you should then be in the form. To get to the form object in the subform control, click on the little box in the subform's upper left corner above and to the left of the design view rulers.