When you install a Fontlab product, the license is linked to a specific computer. If the computer changes, or the hardware configuration of a computer changes, the license won't recognize the new computer and you get an error message.
If you need the app installer, you can find installers here.
If you have your serial number but it is not accepted, you'll need a new license file linked to the new computer. Submit a support request via our online form. In that request please include your serial number. We will be happy to get you up and running.