For uploading via the app to work, the app needs to have permission to store in the phone's settings.
For Android
Open your phone settings
Select "Apps"
Find Storegate
Scroll down to "Permissions"
Under "Permissions", storage should be turned on.
For iOS
Open your phone settings
Scroll down to Storegate
Under "Allow Storegate access" you need to give the app access to the image library.