ok so i had this problem and i saw a post around here dating sincer 2012 that had the resolve for this issue . So what you have to do is :
ctrl+shift+esc while i think you should be at least in a custom game so the game is actually opened (the game not the client)
go to league of legends right click on it and then press go to details , right there you'll have more things containing league of legends in it's name you have to right click on all of them because for sure one of them will have UAC virtualization disabled and you just have to turn that on .
And also you have to Control Panel -> System and security -> Windows Defender Firewall
And then on the left bar of things just click on "allow an app or feature through windows defender firewall"
Then press on "change settings" and then make sure that every league of legends thing there is allowed to do shit in your computer
i ain't an computer engineer or shit i just saw this tutorial in another post and it had the job done for me