Technos are the game's currency that you receive as the game goes on, mostly from doing tasks in the story from either Rad, Enid or Mr. Gar, you receive 6 each day.
For this achievement, you need to have 50 technos on you at once, not just earn 50, so don't go wasting in stuff like cards, just save up until you get 50
Once you unlock access to the Macro Chips, you can sell them to Dendy to get more Technos, as well as selling duplicate cards that you might have.
The game continues even after beating the story, so don't worry.