That depends on what item you are referring to. In general, if you have your account open on two different devices, it could be throwing errors like that, or maybe you used up all of your potions but clicked one twice there at the end or something?
To get to Navel Rock in Pokemon Diamond you need the Mystic Ticket event item. Sadly the Mystic Ticket is no longer available since it was an event item that has long passed.
because it is not available
The Old Sea Map is no longer available to obtain through Mystery Gift. You will need to use a GameShark or an Action Replay to obtain this item and it is only available for Pokemon Emerald, FireRed, and LeafGreen.
That item is only available in Pokemon Diamond, Pearl, and Platinum.
Sadly, this item has been retired and is no longer available.
by getting an event item that no longer is being distributed
The Poke Radar is not available in Pokemon Sapphire. The Poke Radar is an item exclusive to Pokemon Diamond, Pearl, Platinum, X and Y.
yes but you have to have oaks letter and that is an event item which you can no longer get
The Bewitched Pumpkin was a limited edition Halloween BM item. It is no longer available.
The Azure Flute is only available as a Nintendo Event item.
the miracle berry is only available as a mystery gift item
No wild Pokemon hold the item razor claw but you can find razor claw at Route 224, Victory Road and available to buy at the Battle Tower (D/P) -Clensen