You'll want to open the form in design mode and check for the field that is mentioned in the error. Look in the data source task pane at your main data source and see if you can find the field.
It may be marked as 'cannot be blank' yet isn't on the form where the user can fill it out, which can cause errors like this. Also, you may be setting the value of the field with a rule or a default value that is invalid.
When you find the field, drag it onto the form somewhere and preview the form to help you trouble shoot the issue.