The most likely cause is that you were recently in battle or are currently in battle. Wait a few minutes to “calm down” and you will be able to port.

You're carrying too many items. Check the Event tab. Drop some items off at your settlement first. Please do not litter capital cities with dropped items. Litterbugs will be persecuted.

The portal may be disabled or “turned off” by staff. This can happen when a settlement falls or a subscription expires.