How to get rid of “cancels Place Item in Tomb: Item inaccessible”

• Create a squad for them.
• Create a point in the notes menu.
• Station squad at that point.
• Pull lever.

Wall Grates, like any other building, can be removed by query-ing the building and pressing x to "remove building".

Now, you've indicated that this is not actually happening for you.

A couple suggestions: Are you sure the cursor is over the wall grate? If there's another building is closer, it might be latching on that one instead.

If you've successfully scheduled it for removal, is it possible for your dwarves to reach it? Buildings can't be constructed / deconstructed from a diagonal, even though dwarves can walk diagonally.

Have you accidentally forbidden the wall grate's component grate? (Check with t and see if it's forbidden) If a building is made out of a forbidden object, dwarves will defer to the forbiddance rather than the job to remove it (damn, lazy dwarves!).