In order for the app to work please follow the steps below :
The app to Shopify app marketplace https://apps.shopify.com/shopupz-inventory
Click on install the app
Approve the app installation , you will be redirected to approve subscription page
Click on "Approve" button as displayed in the screenshot below. Please note that in order for the app to work you must accept the charge.
You will get the free trial period
Good luck and happy sales :)