If there is no folder for Windows Update, you can create one and then proceed according to the instructions. See this article for reference. How to block the Windows 11 upgrade on Windows 10 PCs - gHacks Tech News
I had to do this with one of my computers and it worked fine. Here is another article that says the same thing. Note Step 3 in particular.
Disable Windows 11 Upgrade Using DisableOSUpgrade Registry Key
1. Type regedit in RUN or Start search box and press Enter. It’ll open Registry Editor.
2. Now go to following key: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows 3. Create a new key under Windows key and set its name as WindowsUpdate. If the key is already present, skip to next step.
4. Now in right-side pane, create a new DWORD DisableOSUpgrade and set its value to 1
