A few days ago, I noticed I couldn't run some applications because of .NET Framework 3.5 not being installed on my system.
I downloaded the installer from https://www.microsoft.com/en-US/download/details.aspx?id=21 but when I tried to run the installer,
It said this:

I tried pressing 'Tell me how to solve the problem' but the error wasn't present there. I also tried to find information of the error code using slui.exe but it said it could not find any information from it. Optional features also doesn't work because it doesn't list a single feature:

mscoree.dll is also missing which means I can't run applications like PowerShell or any files that requires mscoree.dll.
Windows Update and Windows Update troubleshooter also doesn't work.

'SFC /scannow' and 'DISM /Online /Cleanup-Image /RestoreHealth' doesn't change anything either.
What can I do to fix those errors?