Element Board....

Each slot has its own element, so if it doesn't fit is it possible you already have the same one on the board?
Here is the element collection list:
https://www.carls-sims-4-guide.com/collections/elements.php

Some of them are really hard to find. I've only found them doing the Scientist career and using the analysis machines there, so I usually save the board collection for my Scientist Sims.

For the rest of my Sims, I like to send extra crystals and metals to the geo council to get some Elements back in the mail, because I think they make cute paperweight decorations for the Sim's desks. :

You can hover over each element in the board and the tooltip will tell you what type it is. The element board is a good way to keep track of the collection since you can only have one of each element there. There's probably also a section for the Elements collection in the Collections panel on your personal inventory if you find that easier to use.

Mind though, if you have Get to Work, the alien element isn't included in the board, so that one won't fit in there.

Sometimes I find lost items in house inventory.