Hello,
Can I use my serial number from Scrivener 2.8.1 on my new Macbook Pro (10.14.4) ? I downloaded the version of Scrivener available on the site and it says my serial number is invalid. Is this a bug or do I have to buy a new version of Scrivener?
Thanks! 
You need to download the old legacy version 2.8, not the new version 3.
No, a Scrivener 2 serial number will not work with Scrivener 3.
You should probably upgrade. While 2.8 will run with Mojave, it is not supported and there are several known issues. Since 2.8 is no longer being updated, those issues will only get worse over time.
If you purchased Scrivener 2 via our store, the store should recognize your email address and apply the upgrade discount automatically.
Katherine
Thank you to all of you! I appreciate it. I will upgrade pronto. 