I have been struggling with the same thing yesterday (hours of google search, even downloaded servers on both host and guest to transfer files, but couldn’t make it work).
The problem for me that the ISO file is automatically mounted already, and its read-only for some reason when I open it from Desktop (first time Linux user here, so that might be normal) There are suggestions to dismount the drive first then it will start automatically… It might work for you if you try to dismount the ISO, restart and then try to click on “Insert Guest Addition CD Image”. more on that here.
What worked for me:
- I have opened the mounted CD from the Guest desktop (you should see a CD icon there)
- When you are in file manager right click anywhere in the folder and select “open terminal here” option
- Run the command below
$ sudo sh ./VBoxLinuxAdditions.run
note: I have only seen read-only exe-s, but i gave it a go anyway and it worked!
- Type in your password and wait until it installs.
5 Restart your system.
Hope this helps!