@pluto7073 Hello! I too had a problem with urns. Unfortunately, @jpkarlsen is correct. If a sim dies while you are playing another household, there's a chance the urn will not spawn. Did you also check in that sim's household inventory? Sometimes urns will show up there. Otherwise, as far as I know, there is no way to recover the urn / tombstone without using a mod.
If you're on PC or Mac and don't mind using the MCCC mod, then here are the steps I took recently when I missed out on an urn (these are based on the most recent public version of MCCC)
1- Play a sim who has a relationship with the ghost sim for whom you want the urn
2- Go to the relationship panel, click on the ghost, choose the MCCC option -> Sim Commands -> Maintain Sim -> Revive Sim
3- Invite the newly revived sim to hangout, then when they show up use the friendly interaction "Ask to Join Household" (they should maintain all relationships)
4- Once the newly revived sim is back in the household and on the lot, click on that sim, choose the MCCC option -> Sim Commands -> Maintain Sim -> Kill Sim (choose type of death from the list)
I know it seems a bit harsh but they did die already, right? And this way you will have their urn to cherish and be able to keep their ghost around. I hope this helps.
Edit: CAUTION - If you haven't used MCCC before, I should warn you that there are default settings that will change your game; population settings may move sims around, pregnancy / marriage settings may give unplayed and NPC sims babies and marriages. Probably best to do a "save as" before adding any mods to test it first and see how it works, so you don't inadvertently mess up your current save.
