It dones they me add items to pocket storage and I can’t find a config file or something to add them in.

I'm not sure what you mean, you open the item in your inventory, then you can move items to it as you would any other inventory.
And items will automatically get picked up by it if its an item already inside.

Okay. I’m trying to add Timeless And Classics ammo inside the pocket storage. I get this message. Only simple items(no data) are allowed inside the pocket storage.

Okay. Is there any I can do so it can be added?

ah, that means the item has NBT data on it, and cannot be stored in the pocket storage.

Not really, it cannot contain items with NBT data, and thats how the ammo is functioning, you are better off using a bag mod, like one of the many backpack mods, to hold the ammo.

It is possible, in 1.21, depending on how they do the ammo, that it could be held, but i don't know enough about the other mod.