Hi ! This is probably due to adware that is installed on your device .
Go to settings > apps and look for the app , that you might not have installed via playstore and uninstall it .
Further this can also happen due to any app that you have installed via playstore , that shows ads
Such as Free Antivirus or Phone Cleaner or Speed Boosters application , as these apps have higher level permissions so they can show ads without any issues .
If you don't get a solution from the above suggested Method , then :
Settings > Apps and then click on the 3 dots on the upper right corner and then go to Default apps and set everything to Default , further RESET APP PREFERENCES.
