Freezing Header Columns in Numbers

Trying to keep the first two columns of an existing Numbers project visible when scrolling across. The Numbers Help suggests the Format sidebar, table tab and then accessing the Headers & Footer drop-down. The only available option is "0." The numbers below that, 1-5, are grayed out. The other two options there, for top and bottom row headers, have 1-5 selectable. The columns not.

Likewise, in the Numbers menu bar across the top of the screen, pulling down Table gives the option of Header Columns - again, the numbers 1-5 are not selectable - only 0.

I know this must be a basic thing - just missing it.

Appreciate any help!