Check on your dashboard to see if you already have a goodybag active and possibly one queued up to start after the active one ends https://giffgaff.com/dashboard
IF not, then check to see if you have received any messages/emails from agents warning you about your useage on an unlimited data goodybags - if you have been tethering on an unlimted data goodybag or download files for use NOT on a mobile phone, then you may have had a barr placed on your account - these normally last 90 days from what I can gather/read on the forums
Contact an agent using this link to ask why you cannot buy a goodybag (if you do not have ay queued ready etc) - use this link https://giffgaff.com/support/ask/cant_buy_goodybag and explain your problem in the box at the bottom of the form. Agents can take upto 24 hours to reply although often reply quicker - they will have access to your account and will be able to let you know the reasons why you cannot get a goodybag (if it is an account issue)
Hope you get it sorted soon, agents will normally try their very best to help sort it out.