Some files cannot be scanned. the files are usually archive files and are by nature inert. The files are not executable files. I am taking it for granted this is a custom scan. If so, click packers and leave unchecked the all packers box. For more info check out the default scan settings. I had the same problem until I unchecked the all packers box. Since then I have had no problems. Hoped this helped.
Dell Inspiron, Win10x64--HP Envy Win10x64--Both systems Avast Free v17.3.2291, Comodo Firewall v8.2 w/D+, MalwareBytes v3.0, OpenDNS, Super Anti-Spyware, Spyware Blaster, MCShield, Unchecky, Vivaldi Browser and, various browser security tools.
"Look before you leap!" Use online scanners before you click on any link.