"Add column" greyed out in Numbers

I have a fairly complex spreadsheet and I am not able to add (or remove) columns - the option is greyed out

The sheet was created (by someone else) in Excel and imported

I can add/remove rows without problems

There are no header rows or columns, and in merged cells (there were, but I've removed them)

I can copy the whole sheet into a new sheet and then things work fine. But that messes up all the references to this sheet in other sheets so I'd rather fix the original sheet

Any ideas why this happens/how to fix?