You can only split those cells that have previously been merged.

To split cells, select the merged cell and do one of the following:

–On the Home tab, in the Table group, click  Unmerge cells.

–Right-click the range and select Unmerge cells from the context menu.

–When working in macOS, run the Format > Table > Unmerge cells command from the command menu.

–Press Alt+Shift+M (Windows, Linux) or ⌥Option+⇧Shift+M (macOS).

The data that is contained in the merged cell is placed in the upper-left cell of the resulting range. Cells in the range will retain the formatting of the merged cell.