Sold 2nd Home, received 1099S. TT selected Long-Term Box F on Form 8949, but 1099S does not list basis, so why can't I check Box E instead? Override results in an error

You should not override this form. 

TurboTax selected long term based on the date of ownership.  If you own a property for more than a year it is considered long term.  If you owned it less than a year then it would be C instead of F. You should not choose anything else.

Your 1099S would not list basis, your basis is the price you paid for the house plus any improvements that have been made to the house. Box E is not relevant for this sale as you are not entering a 1099-B.

Then if you had other selling expenses like commission, you would click a few more boxes and under select any less common adjustments that apply put a check mark in the "The reported sales price did not deduct all fees or selling expenses"  Then enter your selling expenses. 

If you are trying to enter the cost basis in forms mode, you would need to do it in the Cap Asset Sales sheet.  Form 8949 is pulling the information from the home sale from there.