This message will appear on the screen of your battery if it does not see a connection between the battery and the coil in the tank. For batteries with no screen this is usually shown by the button flashing several times.
Often the fix is one of these things
Make sure the coil is securely in place.
The coil may need replacing
If you have another device try the tank on that as a process of illumination to find what part is causing the issue.