I cannot delete a line 21 entry and subsequently can't close out the return and file an amended return for the same year?

The easiest way to delete a form in TurboTax Windows is to switch to the Forms view (Ctrl+2) and search in Open Form to find the form you need to delete (Schedule C or Schedule E). When the form you want to delete populates on the screen, click Delete Form at the bottom of the screen (if there are any subforms or worksheets associated with the forms, delete them first, then the main form).

did not work.  line 21 entry is a manual NOL entry that I want to delete from the 1040

If you did a manual entry you need to return to the place you made the entry to delete it ... if you did an override in the FORMS mode go there.   In the FORMS mode you can find the unwanted amount and click on it to data source where it is coming from.