My immediate suspicion would be that your messaging app is converting a long SMS to an MMS and you cannot send it because (a) you have no data allowance or are not set up to use data/MMS and/or (b) you have no cash credit to pay for sending MMS messages (30c each).
What phone/messaging app are you using?
Often messaging apps will automatically convert "long" SMS messages (e.g. those that would use 2 or maybe 3 SMS messages) to MMS.
Often there is a way to configure the messaging app NOT to convert "long" SMS messages to MMS.
If not then there are lots of (free and commercial) messaging apps that allow you to prevent conversion to MMS.
Hope this helps.
