By default, the POS logs out the current user after 60 seconds of inactivity.
To turn this off, you need to ensure that the Cashier Auto Logout field is empty.
Go to Settings > System
Scroll down to User Management > B4. Cashier Auto Logout Time
Select the grey field
Increase the number of seconds before logout, or backspace until it is blank (to turn off auto logout completely).
See also: