Configure Sophos Anti-Virus V7.X for Mac
Click on the Sophos Anti-Virus icon and select "Open Preferences..."

=> click and select “AutoUpdate” button
=> Change and set the following settings:

Configure On-access Scanning settings
=> Click the "On-access Scanning" button
=> Check and ensure the on-access scanner is on
=> In the "When a threat is found:" field, select "Clean up threat"
=> In the "If cleanup fails: " field, select "Deny access and move threat"

Configure Scan Local Drives settings
=> Click the "Scan Local Drives" button
=> Enable "Scan inside archives and compressed files"
=> In the "When a threat is found:" field, select "Clean up threat"
=> In the "If cleanup fails:" field, select "Move threat"

Set Sophos to perform update immediately by clicking on the Sophos anti-virus icon in the top and select "Update Now"

Scan all local drive by start Sophos Anti-Virus by clicking on the Sophos icon in the top and select "Scan Local Drives"
For more information and details, please refer to the Sophos Anti-Virus's Manuals for Sophos Anti-Virus help manual and Standalone startup guide manual.