To install an app from appmoon you need to enable 'Unknown Sources' or 'Install Unknown Apps' in your device settings. Android disables the ability to install apps by default.
1) Open the settings app
2) Search for 'Unknown Sources' or 'Install Unknown Apps' and enable.
3) If you can't find via search, within the Settings app, you can normally find it in a section
with 'Security' in the title. e.g. 'Lock screen and security', 'Security' or 'Biometrics and security'