maybe you have been abusing the unlimited data goodybag.
the only people who would know the answer to that are agents https://giffgaff.com/support/ask
they are usually pretty quick but remember an agent can take up to 24 hours to reply
put your query in the tethering drop down
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.