Please check your dashboard: https://giffgaff.com/dashboard
Do you already have a goodybag under 'Active', and a queued goodybag under 'Next'?
If so, then you can't buy another goodybag, and can only add separate 'credit', hence the message
Also *100*7# for your goodybag balance and mention any expiry dates here.
If you already have a goodybag, can you actually use it? Do calls / texts / data work?