does anyone know how to fix this error? I didn't mess with any of the game files.

This is either an issue with your vid drivers or, more likely, you need to install the old DX9 files from XP days (https://www.microsoft.com/en-us/download...x?id=34429); if you have RCT3 on disc, the required files may be there. This will not interfere with newer DX installs, it goes in its own folder and is only accessed when windows cannot find required files in the new DX. (FWIW, between DX9 and DX10 Microsoft changed the organization of the DX files; while new DX is backwards compatible, some older programs are not forward compatible and seek files by older path names thus do not find them in newer DX versions.)

It is not uncommon to end up with several DX installs if you play many older games, they are sometimes coded to only work with a specific version. You can see what you have installed by checking "Programs and Features" for "Microsoft Visual Basic C++" (I have seven installed, two dated 2008, one dated 2010, one dated 2012, two from 2013, and one from 2015).