Considering that the GameJam has this rule :
Assets - Use only assets you have permission for (your own, free, or purchased). If you didn’t create it, you must list it in your submission;
Having things prepared in advance like potential animations, prefabs/systems, codes, basic setups, etc. isn't against the rule.
For those who don't know the term prefabs, it a word that means pre-fabrications and those are packages containing stuff already built which may include script, models, animations, collisions, etc. made for a specific game engine.
You simply have to consider if what you wish to prepare in advance comes into the spirit of a game jam or not. It's not like there's a Jam Police that will knock down your door because you prepared a few things in advance for the sake of making sure such things work once implemented into the actual Jam.
For example, nothing is wrong (especially for people who have never done it) with creating a project earlier to implement basic functionalities like a working settings menu and testing out if you're actually able to export and run the project as something like an HTML5 web project.
This is a gray area in the world of Game Jams that are remotely managed.
The only Dev Jams that have actual limitation with what you can start with are the Jams that require you to be physically in a specific place by providing the PCs, software, space, food, etc. to work on the Jam as it runs. Basically, in those Jams, everyone uses the same tools (PCs + software), have the same environment and more. This is simply impossible to manage with remotely managed Jams because, for example, I might have access to software you don't and can't access to anymore and the one next might have a Desktop PC with 3 screens that can do things a LOT faster and efficiently than the other next one who work from a 4 years old gaming laptop that starts to show its age.