How can I redeem the 50$ offer code of DigitalOcean in my Student Pack?

Recently I requested my Student Developer Pack successfully.But when I try to click the “your offer code” website(https://education.github.com/pack/redeem/digitalocean-50) of DigitalOcean .The website was relocated to the original website which shows a note saying"That offer isn’t available right now, please try again later.":confused:
Can you help me? Thanks a Lot