The problem is actually very simple: in Windows 10 the hosts file is read-only by default. This means that even if you can access it with administrative privileges, you still cannot write to it.

Furthermore (at least on my machine) this means that editing a copy as others suggested won't work, because it will be read-only too!.

To be able to edit it you must first disable the read-only bit, perform the following steps.

Tutorial

Steps

1. Open the c:\windows\system32\drivers\etc\ folder in your file manager,
2. Right-click the hosts file,
3. select Properties,
4. un-tick or un-check Read-Only,
5. Click Apply,
6. Click Continue (to perform the action with administrator privileges) and Then click Ok.

After those steps, you can now freely edit the hosts file with your favorite text editor (running it as an administrator). Remember to re-set the read-only. Watch the tutorial above in case you didn't understand the written steps.