to a more recent version of Parallels (like version 19 or 20), as version 15 may not be compatible with current macOS releases. Parallels Forums Where to Buy
If the key is correct but won't activate, you can try forcing an online activation through the Terminal using the command: prlsrvctl activate-license-online . Old activation key issue? - Parallels Forums
Do not risk your digital safety for an outdated piece of software. Use VMware Fusion (free) or pay for the current Parallels. The time you save not fighting with dead activation keys is worth more than $100.