Errors when attempting to switch branches & when pushing Github

[Cole - Nova - Andrews]

Hey all! 

Alright so super noob here I was attempting to use Sourctree (and github) as a means of source control for my project in Unreal Engine 5. The first couple commits & pushes were fine as I had no large files. 

Eventually once I had some larger files I got the initial stage error/warning saying I had large files, I ignored this as I did want those files to be included - many of them are vital to the overall functioning of the project. However, when I would actually try and push (after staging and committing) I'd get post buffer errors and would ignore the issue (leaving it as a commit pretty much). 

Fast forward to tonight I have three branches: Main, Combat, ALS. Combat is where my previous three commits are and ALS was a new branch I made as I was about to start working on a new system that may or not break a lot of things. I tried pushing per usual and once again got a post buffer error, however this time it recommended I try git LFS, great! 

Downloaded git LFS and seemingly set it up (as I'm new to git I can never really tell), and then double-clicked on my combat branch... wait did I just swap branches? 

Booted up my project and yeah, I went back to a commit when I swapped branches, from what I can tell at least, from many days ago. Now when I attempt to swap back over to the ALS branch where I actually need to do the work, and where all my progress up to now is stored, I get the following error (see image). 

I've tried looking up solutions already but they all seem risky for a noob like me and I'd rather not lose days of work because of a stupid mistake. 

So a couple things I'm looking for: 
1. How do I fix the error in the image? 
2. In the future what can I do to prevent postbuffer/LFS errors? I tried pushing once I had LFS installed but it didn't really seem to do me any good. Do I need to manually choose which files are sent to LFS vs git ?

Sorry for the ramble and thanks for your help!