You can't add items beginning with FF, unless you specifically know the Form IDs for the ones in your save. Let me explain.
Basically, every item, reference, NPC whatever in Bethesda games from Oblivion onwards have a Form ID. So for example, the item gold has three different things you can refer to it as:
Form ID: 00000000F
Editor ID: gold01
Name: Gold
These are what they are as far as I can remember, but it doesn't really matter regardless.
So the Form ID is what the engine internally knows the object as. So gold to the engine is "00000000F".
The Editor ID is only used in the Creation Kit. This is what developers and modders see the item as, it's also what you use in scripts. So in a script where you have "player.additem gold01", the engine translates that to "player.additem 00000000F", and adds gold.
The Name is simply what the player sees ingame, in menus and such.
In scripts you use the editor ID, but in the console you need to use the Form ID. Form IDs are hexadecimal, the first two digits indicate the load order.
00 is any object from Skyrim.esm. So any object or reference that has been made by the developers in the base game.
Every ESM or ESP that is loaded then continues to increase the first two numbers hexadecimally. So your first loaded mod will be 01, then 02, 03, 04, 05, 06, 07, 08, 09, A, B, etc.
FF is a special prefix. It indicates any reference made ingame that is stored in the current save. So for example, a spawned bandit, a new enchanted weapon you've made, ash piles etc.
When you use the console, it will look first at the prefix of the Form ID, look in that ESM/ESP, and then do what you asked with that reference. If it doesn't exist, it will throw an error.
However, objects spawned ingame with the FF prefix can't be used in the console like normal, because they differ in every save. So you can't just go on a site and look for a FF object, because that won't exist on your save. That's why it doesn't work.
Sorry for the long explanation, but I hope it helps
