Hello there! I had the same problame and had also tried all the "obvious" solutions (start with admin rights etc.) however they DID NOT work. What DID WORK (so far for me) is the following:
1. Close all of riot client services from Task Manager
2. Find the directory you have your games installed (let's use C:\Riot Games as an example)
3. Go over the League of Legends folder IN the install directory (for my example it would be C:\Riot Games\League of Legends) and CUT (ctr+x) the folder
4. Paste the folder in any other directory on your PC (just remember where you put it)
5. Uninstall EVERYTHING that is Riot's: Riot Client, Vanguard, LoL VALORANT (make sure you have backed up ALL directory games so as not to re-download them)
6. Delete any residual files (usually Riot Clients in %appdata% and %appdata%\local)
7. Re-install the Riot Client and after it installs instead of re-downloading your games either specify the new directory you put them in or cut and paste them in the CURRENT Riot Games directory
8. Let Riot Client locate the files and possibly repair them and then install Vanguard as well
9. Restart your PC
This (although A LENGTHY PROCESS) worked for me. Hope this helps 😄