Index or Primary Key cannot contain a null value

[getmemoney]

I have a problem with "Index and Primary key" error in attached file, can someone tells me how to fix it. I'm new at Access 2010. I've deal with this error a week.
I forgot to tell you that I was working on the form "FormHoaDon", and I get stuck there.

 

[pr2-eugin]

Hello getmemoney, Welcome to AWF..

I am not sure what your actual problem is. Your description is not clear, when are you facing the problem? What exactly do you wish to accomplish?

If you could elaborate the issue, I believe many of us here can actually look into the problem and help you.. Now it feels like looking for a needle in the haystack..

 

[getmemoney]

Hi Paul,
Thanks for replying me. I'm facing this issue when I run into a next record of CTHoaDon Sub Form. I created a form/subform manually (w/o Form wizard) by created 2 forms separately, then I dragged the form CTHoaDonSubF into FormHoaDon like a subform by subform/subreport wizard. I don't know if I got a mistake at this step or not. I believe that the error came from CTHoaDonSubF because the main form still writes a record even when I got the error message before I go to the next record of subform, and stuck there.