You have the Data Entry Property on both your Main Form and Subform set to Yes!
Try setting them both to No and see what happens.
Setting Data Entry Property to Yes means that you can only enter New Records...and isn't appropriate for a Main Form/Subform set up.
As with a number of things in Access, the name Data Entry Property is misleading...it doesn't have to be set to Yes in order to enter data!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007