Open Command Prompt as Administrator and type the following command:
slmgr /ipk your-product-key
If you have a valid product key, you can try re-entering it. In the Activation
section, click on Change product key
and enter the product key you received with your Office 365 invite.
Run Activation Troubleshooter.
- Go to Settings > Update & Security > Activation
- Click "Troubleshoot" and follow the prompts
You may need to check with your IT department or the source of your Office 365 invite for further details.