Is this when using one of the body builders, such as for the Strat or Tele? if so, sometimes choices aren't compatible.
For example, if you've specified single coil pickup routings on the body, it won't let you choose humbucking pickups later. They trap for that error.
Note that if you want to buy hardware or pickups for a different body, you need to accept the body you've specified, then continue shopping using the drop-down menus across the top of the window. That way, you can get loose parts that aren't cross-referenced to the custom body you've specified.