If you are getting a script error "llDialog: message too long, must be less than 512 characters when you try to send a message", it's probably because you have a lot of older items inside the controller's inventory (the ones you have sent as attachments to your previous messages).
The message you are getting is because the dialog is trying to display the names of your items for you to choose from, but their combined length exceeds the maximum 512 characters that the dialog can display.
Once you remove a few oldest items from the controller's inventory, you shouldn't get this issue anymore. The kiosks display a maximum of 10 recent messages, so there is no need to keep more than 10 latest items inside the controller. If you have less items and still experience issues with this, please try giving your attachment items a bit shorter names.
To remove items from your controller, follow these steps:
- Right-click your controller in-world, select Edit and go to the Contents tab in the Edit window.
- Delete any older objects you have dropped inside the contents, making sure you don't accidentally delete any scripts.