After several months, I have yesterday at last successfully replaced Windows 10 with Linux Mint (Cinnamon, 64-bit), and am well impressed by it as I continue to transfer my digital possessions thither from my old computer. As Avast! Antivirus is apparently unavailable for Linux, I searched briefly and found Sophos' official Web site. Following the accompanying instruction manual (§4) and having first set my account as root, I installed the programme.
Next, I unpacked the tarball (using Mint's built in archive manager, for the given command is unclear to me, a Linux novice) to my Downloads folder. I went within the new folder and clicked on install.sh. Selecting Ausführen [Run] (the rightmost option) closes the dialogue and does nothing; selecting Im Terminal ausführen [Run in Terminal] (the leftmost option) shows for a small fraction of a second the message: You must be root to install/uninstall this packet. Installation aborted. I have verified that the account I use is marked as root from Benutzer und Gruppen [Users and Groups].
What should I do now? I am at a loss.
(I have revised this significantly from the first version of this problem; you may find the original version pasted here.)