Because you aren't looking in the correct section, I said you need to edit the scan settings.
You need to open the Scan settings that you have been running and under the Report File check the option to Generate report file.
My image is of a scan, AvastUI > Protection > Antivirus - In that section you see 3 Scans, Smart Scan (settings for that are in a different location), Run Full Virus Scan and Other Scans, which will open another section for those. Lets deal with the Full Virus Scan at the top of its square there is a cog wheel, that is the settings link for the scan (that is the same when you go to the Other scans section).
Now you are at the point of my earlier attached image, and there you need to select Report File.
I would suggest that you have a good look around the UI to get familiar with where things are.