Start by clearing your web browser cookies, cache and history by following the steps in this https://community.giffgaff.com/t5/Tips-Guides/How-to-clear-your-browsers-Cache-Cookies-and-History/td-p/19483337 thread or you can use a alternative browser e.g. Google Chrome , Mozilla Firefox etc.
Also check out this https://community.giffgaff.com/t5/Getting-started/I-can-t-activate-my-SIM/ta-p/7435412 link if it helps.
On the home page, click Activate your SIM to begin the activation process.
Enter the activation code. This is a 6 digit code which is found on the plastic card which originally held your SIM card. If you lost this code, you can use the 13 or 19 digit code printed on the SIM itself (right before 3Ggg1 or 4Ggg4).
Complete steps on activating your new SIM card can be found here https://community.giffgaff.com/t5/Getting-started/Activating-your-new-SIM/ta-p/3521635
If you have followed the steps above and still unable to activate your new giffgaff SIM please speak to a giffgaff agent to investigate.
To contact a giffgaff agent you will need to fill in the agent form here https://support2.giffgaff.com/app/ask/Getting-Started/Activating-my-SIM/form/
It can take up to 24 hours for a giffgaff agent to respond (agents work 7 days a week from 8am to 11pm).
You will see when an agent has replied in the "Messages: From Agents" box which can be found half way down the right hand side of your "My Giffgaff" page or you can access it direct here https://giffgaff.com/support/questions