in the insert record behavior, you have bound the StartDate database column to an element named "StDate", i'm guessing this is what the original text box was named before you converted it to a date picker.
the Date picker element is named "SartDate" which is causing the problem.
there are 2 ways you can fix this problem:
1) Rename the date picker element to StDate
2) Edit the insert record behavior and bind the StartDate database column to the StartDate form element.