If you are trying to install ShortPoint on Office 365, and then you stuck with this step:
The issue you are facing is that you can not upload the app. It means you do not have the permission to upload apps to the app catalog site.
You need to add the account you are logged in as an admin on the app catalog site.
Step 1: Open Site Collections Admin Center
Url will look like this: https://yourcompany-admin.sharepoint.com/_layouts/15/online/SiteCollections.aspx
Step 2: Locate the App Catalog Site Collection, then click Manage Administrators
Step 3: Add Your Account as Site Collection Administrator. Then click OK
Step 4: Wait for a couple of minutes until the loader disappears. That's it.