@Mollymay We have a few tips to help you get this working!
First of all, are you using Google Chrome to access the Boost website to claim your offer?
Heres a link you might want to try-
Please try using the above link in Google Chrome if possible.
Also, make sure you are logged into your Samsung account on your phone by going to Settings > Accounts and backup > Manage accounts.
We are aware of some users having issues making this claim. To help us investigate this, could you please reach out to our teams on either Facebook or Twitter alongside a link to this discussion (to show what checks you have already done with us here) so this can be raised internally to be investigated?