I used AutoCAD 2010 but could not install it because I turned off Net 3.5. What should I do if I want to re-enable Net 3.5? thanks
You say "I turned off Net 3.5". I find that odd because the description for this build says " Using the latest .NET Framework instead of 3.5. ". That implies that the build doesn't include Net 3.5.
I'm using this version of Windows in a virtual machine. If I look at registry key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\" for this Windows version there is no mention of version 3.5. That implies that this version doesn't have Net 3.5.
Strangely, if I go to Setting and look at "Turn Windows Features On or Off" then there is an option to enable Net 3.5 but it is not enabled. Now I'm not sure if 3.5 is in the build or not. As this is a customised Windows I suppose that the Net 3.5 code could be missing whilst the control options are still present.
I suggest that first you go to Settings and enable 3.5. If that doesn't work then you can download and install it. Google "Install NET 3.5" or "Download NET 3.5". Preferably get an official version from a Microsoft site.
There is a useful article on how to check what version of Net you have on WIndows Central. Google "How to quickly check the .NET Framework version on Windows 10".