After Office365 has been installed, if you get an error message and are unable to activate the product, you will need to do some troubleshooting.
Make sure your Windows 10 machine has all of its updates done and ensure there is no other copy of Office installed on the machine (eg a trial version - go to Add or Remove programs in Settings) as this will cause activation errors.
Follow the steps on this page to resolve activation issues. After following steps 1 to 3, if the issue is not resolved, download the Microsoft Support and Recovery tool from the page. If this does not resolve the issue, continue with the additional steps to resolve activation issues.