It says “Promo Unavailable” or “Looks like you have an existing promotion already”.
Your OpenAI account already has a promotion running on it — a free trial, a student offer, an earlier discount. OpenAI will not stack a second offer on top of one. Check your ChatGPT billing settings; it may be one you have forgotten about.
The fix: claim on an email with no promotion on it — a fresh Gmail works — then sign Codex in with that account.
Do not cancel your current plan to make room. Cancelling does not clear the block, and you lose the subscription you were already building with.
Note that a paid plan on its own does not trip this error. The cause is an existing promotion, not the fact that you are paying.