Warum kann ich nach Wechsel der Gilde nicht beim Territorialkrieg mitmachen?

If you switch Guilds, some temporary restrictions may be applied to your account - those include participating in Guild Wars and the ability to purchase in the Guild Shop.

Before leaving Guild you will see a pop-up warning:

You can check the remaining time in the Turf War tab.

Those restrictions are a necessary measure to prevent Turf War abuse.