For the last few months, I have been reading "How To Reassess Your Chess (I expected at least this book to work), "Simple Chess," "The Art Of Attack," some ending book, "Modern Chess Strategy," etc. I was not practicing at all, and now I have started to play against the Fritz computer, which has a rating of 1450. I am losing most of the time in the opening and blundering in the middlegame sometimes. I have not read any opening books as I have heard that only after a 2000 rating you should memorize openings (I know the basic setups). I just don't understand. People say doing lots of puzzle increases tactical awareness and reduces blunders. But I am still making mistakes with 2-3 move blunders. What did I do wrong? I have only played about 120 chess games sp far.
Perhaps so called theoretical knowledge has no real use.
You should ask why your rating/ability has not been improving.
At only 120 games you are a total beginner. I presume you meant online games. If those were actual OTB tournament games then you need to switch to checkers.
You need to play less and study more effectively. Just playing games will not help you very much and is a very slow way to improve.
You MUST learn two openings COLD no matter your rating. One for white and one for black.
You will need to do effective tactics training. Most online trainers are not done to help you learn efficiently. There are some, not a lot, books that are better.
You need to study GM tournament games. Play over every game by every player. You will get a feel for what good moves are and which are not so good.
Best if you do not play all the games then at least play all the games for W/B that used your preferred opening. None of them did? Maybe you need to pick a better opening.
Learn endgames! If you get that far you will be surprised how many high rated players are clueless about what to do. If you know them then you will beat a lot of players with lower ratings if you get to the endgame. Did I mention to learn openings and play GM games to get a feel for good moves?
After that you need to move to strategy and positional play so you will keep improving. Tactics will help you beat the slash and burn attack at all cost kiddies, but playing good players who do not do that will require positional understanding.
The other answers are focusing on openings but that's not at all the reason why you're stuck at your level. You've pretty much asked your own question: you keep making blunders. No matter how deep your strategical knowledge is, you won't get anything from it if you hang a piece, no matter how badly you'd been dominating your opponent before.
I'd suggest you to stop caring about more nouanced strategic ideas and pay attention to just the "interactions" between pieces. When you're finally able to keep track of who is attacking/defending whom and calculate the consequences of some forced moves (checks, captures, threats...), go back to the more abstract stuff.
The easy solution is to learn a system - i.e., London System; King's Indian Defence; etc. Look through a few setups and weigh the pros and cons. Then, pick 2 - one for the d-pawn opening and one for the e-pawn opening. Play around 50 games with this system to get the hang of it. I'll leave you to work the rest out.