We recommend these best practices when creating a Lookalike Audience to help you create an Audience that will generate your preferred outcomes.
If you are trying to increase app installs, create different Lookalike Audiences for each operating system.
Use all systems instead of Android or iOS for website related conversions that don't specify an operating system.
Select the same location as the Custom Audience source you used for creating a Lookalike Audience.
Select the same placement as the Custom Audience source you used for creating a Lookalike Audience.