You are using Microsoft Office from the Microsoft Store

Using the app version of Microsoft Office from the Microsoft Store will prevent Citavi's Setup from installing the Word Add-In.

Solution

Uninstall this version of Office via the Microsoft Store. In some cases all new installations are installed as "Store Apps" as well. You can try to remove Microsoft Office using the FixIt tool provided by Microsoft here (see "Option 2").

Once uninstalled successfully, install the "regular" version of Office by following these steps.

Finally, install the Word Add-In:

Open the Windows Control Panel.
Click Programs and Features.
Select Citavi, and then click Change. The Citavi Setup starts.
Click Next, then select Modify and click Next again.
Click the symbol next to Word Add-In and then select This feature, and all subfeatures, will be installed on local hard drive.
Click Next and continue through the Setup.