@rainbowfoxes67 Do you own the Cats & Dogs expansion? It's required for much of the My First Pet content to show up in-game.
If that's not the issue, clear Origin's cache, restart your computer, and repair the game in Origin: open your game library, right-click on the Sims 4 icon, and select Repair. Here's how to clear Origin's cache:
https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/
If that doesn't help either, then yes, you can delete and redownload the pack. My First Pet is the 14th stuff pack, so its folder is SP14, located inside the game's program files. Unless you changed the default install location in Origin, the full file path will be:
Program Files (x86)\Origin Games\The Sims 4
You can delete the SP14 folder, clear Origin's cache again, and then redownload the pack.