I suggest that you try add it to the Quick Access Toolbar: Click File tab> Options> Choose Quick Access Toolbar>then click the drop down button under Choose commands from and look for All Commands> Now look for Browse Office Store, and then click Add>>Once
done click OK.
Now check if the Store is at the top left of your Outlook app.
If not, please share the following information with me to further analyze:
1. A screenshot showing "I see the icon (on the right under "Home")".
2. A screenshot by clicking "File" as the image below shows.