I am struggling to build a good opening repertoire. I have tried a few chessable courses. None of them have worked. My opponents counter my opening pretty easily. What is the best way to build a good opening repertoire? My openings for black are the Modern and the Dutch and my opening for white is the London System?
Expanding on RemcoGerlich's comment...
Beginners often overestimate the importance of opening theory. If your rating is below around 2000 or even somewhat above) it is perfectly possible to get by with a very rudimentary knowledge of opnenings. It is unlikely that you lose many games because of the opening. All of your mentioned openings tend to go for closed positions and are not full of tactics or traps that you can fall into. It is much more likely that you lose for other reasons than lack of opening repertoire.
Much more important is being good at tactics and having a good positional understanding, knowing the typical plans in certain pawn structures. The first part tactics is easy to train by repetition solving puzzles. The second aspect (positional understanding( is harder and requires time. I recommend to read through annotated games or (nowadays) watch commented games of which you find plenty on chess24, chess.com, youtube, etc.
Once you have a decent understanding of plans and positional play, learning openings will make much more sense because you will understand why a certain move is made and also what to do if your opponent makes a non-standard move that is not in your repertoire.
Most definitely you don't want to learn openings by heart without understanding the ideas behind them.