You need to fill out the bundle identifier in File>BuildSettings>PlayerSettings>Other Settings>Identification.
The field will display something like “com.examplecompany.examplename”
For this, you fill out the parts as required: com.PutYourCompanyNameHere.YourAppNameHere
For example, if you company is called GameCompany, and your app is called NewGame, then you put this: com.GameCompany.NewGame
Hope it helps.