Hello @Kyrkke , Welcome to the forums!
I recommend this tutorial, which explains in detail how to properly handle the Load & Save system.
This one is also well explained and covers UI as well.
There are also other tutorials in the Learning section that might be helpful to you.
Also, just to be sure, make sure you’ve assigned your Game Instance in Project Settings, as mentioned in this post.
As a tip, start by building a simple system ,for example, saving just one statistic, and test it step by step, you can use breakpoints (select a node and press F9 to check if it’s being called). Once you have the basics working, you can expand it, and it should work correctly. If it doesn’t, this approach will help you pinpoint what’s not working.
I’m also leaving the official documentation here, which might be useful.
You can also switch between C++ and Blueprint.
If you have any other questions, don’t hesitate to ask again in the forum.
Hope it helps!