If you’re running Office 2016, 2019 or above, you may need to update Microsoft Word. To do that, please open Word. Then click "File", then "Account", then "Update Options" and then "Update now". You may need to restart your computer.
If you still can't find the Office Store, it may be related to your document. Office add-ins aren't compatible with DOC format (Word 2003 and below). The store may be disabled if you're working with them. You can upgrade the document to the latest format by clicking Save As and choosing Word format (DOCX). You may need to restart Word after that.