If your mobile device doesn't have screen lock enabled, you will be seeing this warning message. This means that PIN login, session lockout and autologin features will not be working on your device. 

PIN login - if the device is shared among multiple users (for example staff in the warehouse), it is possible to login to Linnworks Mobile using a 4 digit PIN instead of username and password. 

Session lockout - if the device is shared and the application is configured for multi-user (with PIN login), everytime the device is locked (power button pressed), application switched to another, or screen locked, the current session will be locked as well, and the user would have to enter their PIN to get back to Linnworks. This is very useful to ensure staff who are using the device always login using their own credentials. Good for audit trail - know who have done what and ensures different users with different permission levels can share the same device.

Autologin - if its a single user device, whoever logs into Linnworks Mobile will remain logged in. The actual session still technically expires (for security reasons) but the application can automatically log in once you open the it again. 

Also See:

How to configure PIN login