The serial number you are using might be activated on another computer. Adobe Software allows you to install/activate on two computers. If you try to install it on third computer, you get error "Activation limit reached for [product]. This serial number has already been activated on 2 computers" or "Maximum activations exceeded".
In that case, you need to deactivate the software from the computer on which it is activated. It will free up the activation so that you can reactivate on other computer.
Steps to deactivate the software from your computer:
Make sure your computer that has the software installed is connected to the Internet.
Open Elements Editor (NOT ORGANIZER).
Choose Help > Deactivate.
Always remember that you need to deactivate the software before you uninstall from the computer or format hard drive or OS. See document on Activation, Deactivation | Adobe Products.