Whenever I run the command "/summon minecraft:item ~ ~ ~" in any form ("/summon Item...", any coordinates, any or no tags specified), I get the error message in the chat that says "Unable to summon object." However, I can still drop items, see them appear, and detect them as entities through commands.
You have to provide the item with relevant data. You must also specify a
Count
of 1 or higher for it to be a valid item: