Another subform problem- URGENT

Hi

I have another subfrom problem. Using form/subform in access 2000. Backend is sql server 2000. i have an INST_SSN combo and i need to select a value from this, based on which child records are displayed on the child form.

but i am not able to select a record from the INST_SSN combo. it gives me a message

"Form is read-only because the unique table property is not set"

does anyone have an idea what this means......:-(