SOLUTION POUR CS5 ILLUSTRATOR ET PHOTOSHOP

your os isn't the problem.  the problem is the serial number.

a screenshot of the message you see after entering the serial number would be helpful.

@pbpictures2 As @AxelMatt mentioned, If you cannot deactivate the previous license, then you are unfortunately stuck. Adobe no longer supports CS5 and Customer Care will not extend license activations. Offline activation will not work as it still requires you to have deactivated the previous install prior to attempting to activate the new install. 

What the exact error message that you get when you try to activate you CS5 product?

Try the offline activation for your products as described as Solutionj 2 in the linked document.

https://helpx.adobe.com/creative-suite/kb/serial-number-validation-launching-cs6.html

But be aware: if you have reached your activation limit and you don't have any chance to deactivate an older installation you're out of luck. Adobe has stopped the support of CS - CS6 and the resetting of the actiovation counter isn't possible anymore.