“Your plan is in use on another device”

This means your plan's PC (or phone) slot is already taken by another device.

• Forgot to sign out the old one? Remove it under My account › Overview › Devices, then sign in again.
• Don't recognise a device? Press “Sign out all my devices”, change your password, and turn on 2-step sign-in.

Plans are for one person. A plan used from two places at once is flagged, and plans that are shared can be turned off.