You cannot rotate text in any cell of a table. There are a variety of technoques to work around this limitation:
1) rotate a text box with the header text in it:
2) copy the desired header text (which is rotated) then open Preview and create a new document from the clipboard. Then use the rectangular selection tool to surround one word, then copy, then paste that into one of the cells in you header:
Create new Preview doc from the clipboard contents:
Now copy words out of preview using the rectangular selection tool:
now select the cell where you want this text:
There are some other ways (which you can find by searching this forum).
Hope these help
Be aware that inserting text as background fill, or overlaying your table with a text box, will achieve the visual effect, but the values or text you apply that way will not be avaiable for use in making calculations or charts.
You can still have values in those cells of the table, and you can use that data in calculations and charts, but that will be an extra step.
I created this NCAA Bracket in Numbers last March and exported it as a PDF for easy open and printing from any computer. As you can see, I was able to rotate the text in cells along the edges showing where the first round games were. I didn't use text boxes or any other programs yet now, I cannot remember how I did it! When I opened the Numbers file to update it for this year's tournament, the cells are not rotated. This isn't really a question, I'm just flabbergasted as to how I did it and what the heck happened to it.