Can't place my gravestone from household inventory

I also had this problem, but it's temporary, although scary. It seems to happen when the sim dies while you are not playing that household.

Steps to "fix" or really to just see if it's temporary.

1. Save your game.
2. Go into build mode and place the gravestone where you like it. It will disappear.
3. Go into live mode. Still gone.
4. Unpause the game. After a few seconds, the gravestone or urn will bobble up and down like it's possessed, but it will reappear.

If #4 was true for you, then you're good and you can go back into build mode and move it around all you want. If it's still gone after step #4, no worries. Just don't save the game and exit out. This will restore you to the point you saved at #1 and you might have to take more drastic actions.

More drastic fix.

1. You need the MCCC mod installed.
2. Play as any sim who knows the deceased and summon world sim to you using MCCC.
3. Choose the ghost of the tombstone you're missing.
4. Shift click to add to household. This is necessary so they don't leave and you can get them on the lot.
5. Use MCCC mod to revive ghost.
6. Make sure to move the sim onto the lot's buildable space or the rest won't work.
7. Use MCCC mod to kill the sim and select the method (most likely old age, but you can pick any).
8. Witness the death.
9. The new urn or tombstone will appear.