@tavomelo please try posting your comments in English, that way more people will be able to understand you and possibly provide a solution.
on Android Oreo there's no global "install from unknown sources" button. instead, you'll need to grant the permission to install apps depending on how you get your apk files. for example, when you navigate to the apk file using a file manager app, you'll have to grant the permission to the file manager. if you download an apk file using browser, you'll have to grant the same permission to that browser, and so on. you can read more about it here: https://android-developers.googleblog.com/2017/08/making-it-safer-to-get-apps-on-android-o.html?m=1
