I have activated FDTD already. How come still come out this error? and cannot access the software.
You have 2 errors,
Activated the license for FDTD Solutions and DEVICE - you only activated the licenses for your Zemax and Stack.
Please do not changed or edit anything on the configuration or license files. If you have done this, kindly revert back your changes. Otherwise, completely remove the product software installations including the installation folders. Refer to the KX post below for complete removal process.