The primary datum feature should generally be fairly large regarding the size of the part. When referenced in the predominant datum reference frame, the primary datum feature constrains the location and orientation of the part about a datum feature simulator. If the datum feature is not sufficiently large, the constraint on the orientation of the part may not be sufficiently stable or repeatable.
For example, the datum in the following image causes this error to display.
If you see this advisor message, consider setting the primary datum on a larger feature. In this example, the bottom planar surface of the part would be the best choice as the primary datum feature.