Welcome to the forums, @FUN2291! Follow the steps below to try and reset the entire save and to attempt to fix it.
1. Go to CKAN
2. Delete all the mods in your RSS/RO/RP-1 save to make it pure stock. (Delete through CKAN)
3. Check your GameData folder and make sure only the Squad folder is there (SquadExpansion should also be there if you have a DLC like Making History)
4. Next, go here:
https://github.com/KSP-RO/RP-0/wiki/RO-&-RP-1-Installation-for-1.8.1
5. Follow the instructions. (STOP FOLLOWING THEM WHEN YOU REACH ADDITIONAL MOD INSTALLATION)
Note: The mods in the additional mod installation list aren't required to run RSS/RO/RP-1.
6. Search on CKAN for a SpaceX pack (Probably KK's SpaceX Pack) and include the Realism Overhaul (RO) configs with it as well.
7. Check if you can get KOS on CKAN.
That should be it!
AeroSky's post in KSP IS STUCK PLZ HALP!!! was marked as the answer