This is an array formula, so instead of pressing ENTER when you have
typed it in (or subsequently edit it), you have to press
CTRL-SHIFT-ENTER. If you do this correctly, then Excel will wrap the
formula with curly braces { } - you should not type these yourself.

So, double-click on the cell as if to edit it, then CTRL-SHIFT-ENTER.
You can use <copy and <paste in the normal way once you have set the
formula up correctly.

Hope this helps.