This is a known issue at the moment, further updates can be found here:http://community.giffgaff.com/t5/Service-Updates-N
Try clearing your cache and trying again in the mean time..
I'm trying to buy a goodybag but keep getting the response you can only top up credit at this time and to purchase a goodybag you must try later. This has been happening since last night. Any ideas?
If you have a goodybags due to expire then you'll have to wait until tomorrow to buy one. Please read here to see how goodybags work: http://community.giffgaff.com/t5/Learn-giffgaff-To