If you want to enable it then you have to do :-
- Login to your LinkedIn account and then navigate to Privacy & Settings,
- Now at bottom click on Accounts tab and then click on Manage Security Setting,
- There you will see that Two Step Verification is turn off, just turn it On
- It will ask your phone number, just provide it and register your phone.
- On your next visit LinkedIn will send a verification code on your mobile
- LinkedIn will do this once per device.