How can I enter my license key in adobe illustrator cc 2015?

This is the only legitimate way to get a "verified" copy of Illustrator CC 2015. You pay for the subscription, then install the historical version legally.

: A powerful, open-source vector graphics editor available at Inkscape.org. Vectr : A free, web-based vector tool found at Vectr.com .

Even if a serial number appears to work initially, Adobe’s background processes (like the Genuine Software Integrity Service) will eventually flag the installation as non-genuine and disable the software. The Risks of Using Pirated Software