If you’re getting an invalid product key error, try these solutions.
Note: If your copy of Office came with a PIN instead of a product key, go to http://www.microsoft.com/mac/getkey and enter your PIN to get your product key.
Sign in with an Office 365 subscription
If you have an Office 365 subscription, you can’t use your product key to activate Mac 2011. Instead, sign in with the email address that’s associated with Office.
Open an Office application and click Activate Now.
In the Get Started wizard, click Sign in to an existing Office 365 subscription, and sign in with the email address that’s associated with Office.
In many cases, updating Office will fix the problem.
If you can, open an application like Word and click Help > Check for Product updates.
If you can’t open an application, go to http://www.microsoft.com/mac/downloads, scroll to the to the Office for Mac updates section, choose the latest update available. After the page refreshes, scroll back down to the Office for Mac updates section, and then click the Download Now button.
Get an app password for two-step verification
If you have two-step verification turned on, you might get an invalid product key error because Office doesn’t prompt you for the extra security code during activation.
To fix this, get and enter a unique app password to sign in.
Sign in to your Microsoft account.
Under App passwords, click Create a new app password.
Note: If you don’t see App passwords, you don’t have Two-step verification turned on so it isn’t the reason you received the invalid product key error. Skip to the next section to fix the error.
3. Click Copy app password to Clipboard.
Open Word, Excel, or PowerPoint and when prompted, paste the app password instead of your Microsoft account and click Sign In.
If you don't want to reenter an app password each time you sign in to an Office app, select the Save password in my Mac OS keychain check box.
Check the name of your hard drive
Make sure the name of your hard drive doesn’t contain spaces, characters such as a slash ( / ), or commas. If it does, rename your hard drive and then run the Office setup wizard.
Remove previous Office files
Remove any previous Office licenses, preferences, and subscription files that might prevent Office from activating.
Click Go > Computer and then double-click your hard drive.
Tip: The default name will be Macintosh HD.
Click Library > LaunchDaemons and then drag com.microsoft.office.licensing.helper.plist to the Trash.
Note: If you don’t see the Library folder, it could be hidden. To display this folder, hold down the Option key while you click the Go menu.
Go back to Library, click PrivilegedHelperTools and then drag com.microsoft.office.licensing.helper to the Trash.
Go back to Library, click Preferences, and then drag com.microsoft.office.licensing.plist to the Trash.
Go back to Library, click Preferences > ByHost.
Click the down arrow next Settings > Arrange By > Name to arrange files and folders in alphabetical order and then drag all files that begin with "com.microsoft" to the Trash.
Run the Office setup wizard again.
Remove Office from your computer, and then reinstall
Follow the steps to completely remove Office and then reinstall Office.