This issue occurs if an OEM version of Office has been preinstalled on the system by the system manufacturer and has not been removed completely prior to installing the Volume License version of Office 2016. It only applies to Windows computers.
This work around has been documented by Microsoft at the link below.
After installing a Volume License copy of Office 2016, the screen keeps prompting for Activation.
"Let's get started"
Registry key information
Important: This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows
To resolve this problem, export the following registry keys and delete from computer.