Go to the Customer preview website and log in: http://www.microsoft...b/officepreview
Then, you will see how many PCs Office is activated on. You are allowed use office at a maximum of 5 PCs. In the screen you will see which PCs Office is installed and activated on and how much activations are left.
If you have zero activations left, deactivate the PCs you are not using office on and try the activation again from the PC you are currently using.
If you happen to multiple activations on the same PC, deactivate all and uninstall and reinstall office. Sometimes, when office fails to install properly, it activates on the same PC multiple times and causes problems. Again, deactivate, uninstall, and reinstall to fix the problem.