@buddhachick, yes, I also had problems finding the localthumbcache.package file the first time I looked for it! 😃 It is good to delete that from time to time (and necessary to delete it often if you play with Mods or CC).
Hopefully it is just your one Spellcaster that is glitched (or maybe the lot) and not the save. But weird things like this can happen, and are more likely to happen in older saves that have seen a lot of gameplay. Still, I have seen glitched Sims in brand new saves as well! 🤨
I hope her daughter can create the potion for her to drink! 🤞 But if she can't , I would consider using her Aspiration Points for an Aspiration Reward (it is found in the Aspiration Reward Store panel on the same tab as her Aspiration. It looks like a little gift icon with a bow). Then you can scroll down and get the Potion of Youth for 1,500 Aspiration Points. 🙌 It will reset a Sims life to the beginning of his or her current stage!
And the Copypasto spell on the existing potion is also a good idea! 💡
Feel free to update me on if her daughter has the glitch or not! I hope everything works out for you! 🤞 😇