A key for an older version (e.g., 2018) will not work for 2021.
If you have already purchased the software but lost your key, you can retrieve it through these official channels: How do I redeem and activate my Acronis True Image license? Acronis True Image 2021 Serial Key
The 2021 version was one of the last to offer both and subscription-based models, which can be confusing. True Image 2021 Perpetual License? - Acronis Forum A key for an older version (e
For digital purchases, the key is sent to the email address used during checkout. Common Activation Issues Acronis True Image 2021 Serial Key