Which platforms are popular with which target audiences?

You will occasionally get messages about matches or mismatches of target audience and platform you have chosen. Your choice of audience has a minor effect on the review score, it primarily affects the number of sales your game will generate. The higher the sales modifier of the selected platform is, the more sales you will get with that target audience

Here is a chart of well-fitting and non-fitting platforms for each game audience (notice that Gameling is a non-fitting platform for the 'Everyone' audience):

(The numbers after the platform name represent sales modifiers, well-fitting platforms have sales modifiers greater than or equal 100% and non-fitting platforms have modifiers less than 100%)

^{Source: Game Dev Tycoon wiki, "Game Development" article}

There are many different combinations that can result in a 'bad target audience' or similar. There is only a limited amount of information available this close to the release of the game but most of it can be found on the Game Dev Tycoon wikia page for Game Development.

In your particular instance, you targeted a Sports game at a platform (Gameling) best suited for a younger audience and I think this is what triggered the 'bad target audience' message in your question.

Sometimes, it's not about the platform but the game type and genre that can cause the bad target audience message, as detailed in the Genre/Platform combinations table on this page.

In addition, certain combinations of game work better when targeted at specific audiences, for example Post Apocalyptic/Action and Post Apocalyptic/RPG games are best suited when targeted towards more mature audiences.