As we've double checked with our database, your license had no activations left and for security reasons, our activation server allows you to activate the license only a certain number of times (5 by default). We've just added additional activations for your license, please try the activation process again. Just take in mind, it shouldn't be performed many times in a short time period. Thanks for your comprehension.
Let me mention, that the code remains the same all the time. Feel free to reuse the license code you received with the software, it's valid forever. Also, any time needed you can use Resend keys button under User Panel - My Licenses. Usually, the license activation needs to be repeated just in case of re-installation of operating system (OS) or CPU / BIOS change (change of GPU shouldn't affect it, neither change of 3ds Max version).