Why can you not use an 8 hrs shield straight after the expiry of a shield?

The game doesn't allow you to stack shield timers so if you already have a shield active then it will give this warning to let you know that you will cancel the current shield and replace it with the one you are activating.

If your shield had already expired then this must have been a glitch with the game thinking you still had one. If so just ignore it an activate the shield anyway.

The only time you cannot activate a shield is during a cool down that starts if you cancel the shield by attacking someone. You have to wait 1 minute for each level of your base (eg Level 25 base = 25 minutes cool down) before you can add another shield.