Welcome to the community, Let's try to sort this out.
What happens when you try to use 'Select sky' or 'Select subject'?Do you see any error messages? Does this happen with specific file formats?
Please confirm that you're using the latest version of Lightroom for Android from the Google Play Store. Additionally, check if your device meets the minimum system requirements for running the app smoothly: Details: https://helpx.adobe.com/lightroom-cc/system-requirements.html.