The exact message depends on 1) whether you are in turn-based mode or real-time mode and 2) whether there is a floor above it.
Hover the cursor over the item.
If there is no floor above the item, you get a "cannot reach" message in either mode.
In turn-based mode, if there is a floor above it (and a path to that floor) you get a "not enough movement" message.
In real-time mode, if there is a floor above it you (and a path to that floor) your character will try to reach the floor above the item. If it works, you can access the item.
