An activation code can normally only be used once. Once the profile has been downloaded to a phone, the code is spent, even if the profile was later deleted.
Has it already been installed?
Check your phone's cellular settings for a SimFuse line. If it is there, the code did its job and you do not need to install anything. Go to "My eSIM is not working: start here".
Did you scan it on another device?
A code scanned on a tablet, an old phone, or by someone you forwarded it to is used up. Only one device gets the profile.
Did you delete the eSIM?
Deleting removes the profile and does not return the code to an unused state. This is why deleting an eSIM should be done deliberately. See "Deleting an eSIM, and when not to".
Did you try to install it twice?
A failed first attempt can still have consumed the code, which is the frustrating version of this error.
What to do
Contact us with your order ID and your ICCID, and tell us whether the plan ever connected. If the plan has not been used, we can look at reissuing it rather than leaving you with a code that cannot be redeemed. See "Get a replacement for a faulty eSIM".
Not this error?
If the wording is "Cellular plan cannot be added" instead, the cause is usually different. See that article.