If your GitHub for Startups benefit indicates "Already Redeemed" but you did not receive a coupon code or redeem URL, it is likely that the credits were previously allocated to the Microsoft Account (MSA) associated with your email address, possibly from another Microsoft program. To resolve this issue, you should contact support at https://aka.ms/startuphelp-mfs-portal for assistance with activating your GitHub for Startups benefit credits.
If you are still using credits from another program, it is recommended to finish using those credits before attempting to activate the new ones. Alternatively, you can set up a new MSA with a different email address to use credits from two programs in parallel, but keep in mind that workloads cannot be shared across MSAs.