I have issues with Adobe Production Premium CS6 I can't activate because the serial number has been use on two old computers before but I can't access the old computers to disactivited

Bottom line:

If you don’t have access to the old computers, you can’t deactivate to free an activation.

adobe won’t be able to help.  see https://community.adobe.com/t5/download-install-discussions/activation-limit-reached-for-non-subscription-adobe-desktop-applications/td-p/14804413?linkId=100000305463075 

The only way to deactivate is to have access to the old computer. I am afraid there is no other way out of this. You could try connecting with Adobe Support