sammitrev Maybe you are trying to send an MMS, rather than an SMS text?
Not all calls and texts are covered in your goodybag allowance. 01, 02, 03 UK landlines and standard 07 UK mobiles are, everything else requires credit. Here's a link to top-up, if required http://www.giffgaff.com/buy.
More information about allowances included/excluded from goodybags can be viewed through this link https://giffgaff.com/help/articles/inclusive-and-non-inclusive-calls-texts-and-services. Thanks.