Enable app install from Unknown Sources to install apps from a third-party app other than Google Play Store.To do that go to Settings and search the word unknown– For Android Nougat or lower version click on Unknown sources and enable that setting.
– For Android Oreo or later, clicking on Install unknown apps which will take you to a list of apps. Select the actual app through which you want to install the Head Soccer mod app.
Or you can do it other way: Let’s say, you have downloaded the Mod APK on Google chrome or firefox browser on your phone.
Go to Settings > Apps & notificationsor go to Settings > search for unknown > Open the app (Chrome, Firefox or file manager app etc) > Install unknown apps and enable Allow from this source
See the below screenshots that enables to install unknown apps on Oreo for Chrome app (you need to enable on the app from where you want to install the apk):