Basically what the title says. I had a sim die on a lot just for the plot- I didn't want to play with this character, but I did this so I could have the other Sims get a memory that this sim died. After that, I moved the grave into one sims' inventory because I didn't have a cemetery in my game yet.

Today I finally built my cemetery so I'm moving some graves there, but I can't move this one, I get an error everytime. I tried mourning as well, however, any interaction with the grave makes an error pop up and I have to reset the sim. The same happens with any other sim that tries interacting with it. The error is "hit break point primitive".

I've heard graves are usually an issue in apartment lots- and this sim is on an apartment lot, maybe this is adding to the problem. Also, the sim that has the grave in their inventory is old, as long as I know, there's no danger of neighborhood corruption if a sim has a grave in their inventory, but there might be one if that sim with the grave dies.

It's never a good idea to store graves in inventories. It's better to use the inventory to just transport the grave to the cemetery lot, and leave the grave there by clicking somewhere on the ground. It's then possible to visit the cemetery lot again in buy mode, to rearrange the grave or decorate around it.
Have you tried having your sim carry the grave to the cemetery that way, does it also throw errors?

Also you seem to have several bugs which really indicates that a spring cleanup might be a good idea. It's an old game and thousands of mods have been created over the years, many of which have similar functions and affect the same game features. That's the main cause to the bugs we experience, and the only way to have a pleasant, bugfree playing experience is to either use very little CC, which is impossible for most of us, or to be careful when adding mods, adding them one at a time and playtesting a bit afterwards, before adding other mods.

It's possible there's some (hopefully minor) corruption going on with that one sim/gravestone.

The below will only will work if you can move the gravestone out of the sim's inventory, but not interact with it without getting errors).
(Backup your game firs, just in caset!)
1) If possible, move the gravestone out of their inventory and place it somewhere on the lot. Not sure if apartment lots are safe for this, though.
2) If you don't want to take the risk of an apartment lot, make a small empty lot on your map, make a sim (one you can just let live there, they'll probably show up as a townie from time to time), then use a mod like the SimBlender and spawn in the sim who has the gravestone in their inventory. Temporarily make the sim selectable. Place the gravestone on the lot. Make the sim unselectable and tell them good-bye.

I don't know if this will work, but this way the gravestone is still in your game, it hopefully isn't hurting anything, and it isn't making any problems on other lots. You'll want to keep that sim there, so don't move them out.

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Just a tip, but if you have a community lot you use as a cemetery, you should be able to click the gravestone/urn and select "move grave" which will move it for you (will appear next to the phone on selected lot).

There's more info here: https://sims.fandom.com/wiki/Gravestone

The inventory method can be a bit risky, especially if you forget sims have graves/urns in their inventory and they die unexpectedly, which can cause corruption for the sim that grave/urn belonged to (they can become unlinked).