Have you selected the Table?
Copy will be greyed until something is selected.
"I can't find a way of selecting just some columns…"
Numbers does not support selection of non-contiguous cells. You should be able to select columns C,D and E, but would not be able to select columns C,D and F.
To select a set of three contiguous columns CDE for example):
Click on any cell in the table to make the table active and show the row and column reference tabs.
Click the reference tab for column C
This will select all cells of column C.
Press and hold the shift key, snd click the column reference tab for column E.
This will add column E and all columns between it and the previously selected column C to the selection.
Pressing Command-C at this point will copy the selected cells.
Going back to the bigger situation: "I can't find a way of selecting just some columns from the table to print"
If the columns you want to print are a contiguous group, you can do this:
Select the first column tab of the set, then shift-click on the last column tab of the set.
With all cells in the group now selected, press command-C to copy those cells to the Clipboard.
Launch Preview.
In Preview, go to the File Menu and choose the first item: New from Clipboard.
Preview will now display a page containing a pdf image of the cells you selected.
You can print this directly, or copy the image and paste it into a Pages document and print it from there.
Regards,
Barry