Jean Chodz'ko,

Take a good look at the system requirements for the trial version.

You have not told us which trial version you have downloaded, but I suspect that it requires Mac OS X 10.12 or later.

A 2009 MacBook might not be able to run Finale 26—depends when it was built. The early and mid 2009s are capped at OS 10.11 (El Capitán). The late 2009 MacBook can be upgraded to  OS 10.13.6 (High Sierra). You can look up your machine here:

https://everymac.com/systems/apple/macbook/index-macbook.html 

If you have the late 2009, there's a lot of life left in the design with replacement batteries, solid state drives (highly recommended) and up to 8GB RAM all readily available and easy to install. A word of caution: when these batteries get too old, they expand and crack the motherboard (been there a few times).