in Numbers, hitting enter key creates a new row instead of moving the cursor to the row below

in Numbers, hitting enter key creates a new row instead of moving the cursor to the row below.

In addition, the row below, in the drop-down menu on the far left by the row number, "add row above/below" is missing and the only option is "add footer row above/below". There's something wrong with this row, and it is interfering with the sort function: even after sorting, that row is not in the correct position.

I can just scrap the document and start afresh, but am curious as to why this is happening.