Problem with table

I’ve created a table in project notes to use as a calendar. Somehow, in cutting and pasting events, two cells got merged–so the row for one week in my calendar has only 6 cells. I can’t figure out how to get that cell back. Cmd-Z does not restore it–it goes changes I had made in the table and back to changes in the main document without restoring the deleted or merged cell. (I say merged, because data from the missing “Wednesday” cell is in the “Tuesday” cell.)
Using Scriv 2 yet, on Mac Sierra

PS: When I go to Format->Table->Table, a dialog box opens. It has a “split cell” option, but that is grayed out, unresponsive. Selecting the material in the problematic cell does not un-gray the option.