On the Xbox one the type-10 defender cant equip a 7A power distributer despite having a level 7 slot for it.
check your other equipment, thrusters for instance, you may have smaller thrusters, if the increased mass of the PD makes the ship go over the max mass the thrusters can handle you won't be able to equip it.
This

. Post your complete loadout (e.g. as a link to
Coriolis) then we can see what might be going on.
Also, it might be helpful if you told us how that "can't" manifests itself - ideally a screenshot of the outfitting view when you select the 7A PD from the outfitters.
Using the Type 10 or Corvette or Cutter for that matter it comes with a base Class 6 shield even though (excluding the corvette as it's max optional internal is a 7) they have class 8 slots available.
Optional modules are optional - you don't need to have a shield at all. In my Cutter, I fly the shield in one of the size 6 compartments and use the two 8s for cargo. And quite a few of the ships come with the cheapest and smallest module that will do the job as a default, even if the compartment would allow for something larger. Take again the Cutter - default is a 6E PP in a size 8 compartment and also a 6E FSD in a size 7 compartment. Default loadout is just the bare minimum to move the ship from the showroom.