In Purchasing you must have permissions in user setup to override format control options.  If you do not have permissions setup correctly, have your Purchasing Administrator do the following:

1. Go to Tools , User Settings.
2. Choose PO user.
3. Click on Options.
4. Select the Format Controls tab.
5. In the Purchase Order Format section, check the "Allow Override" box.
6. Click [OK].
7. Click [Finish].

The next time you print a purchase order you will have the option to choose your format and print the purchase order.

Refer to the attached article below for steps to setup security in Purchasing or Inventory.