When I try to activate the game on steam, it shows this...

Please first activate the original game

The product code you've entered requires ownership of another product before activation.

If you are trying to activate an expansion pack or downloadable content, please first activate the original game, then activate this additional content.

Here is a screenshot of Rising Storm 2: Vietnam I bought from GAMESPLANET:
https://prnt.sc/vwop4j

Oh oops... I bought the deluxe edition addon of the game, not just the game...