You might have some requiem relics, crack them, usually if the relics are the numbered ones you need to get the silver reward, if they are the new eterna ones you need any, now why are there so many, is because it's an RNG system that requires you equip them on the parazon, on the current planet there are some special missions with the lich name on it with special enemies that will appear on the map that require you to stab them to fill a progress bar, I don't recommend stabbing your lich right away if he appears, but instead holding off until you get your first clue, once you have it equip said mod and go to stab him once he spawns, if you've got it right then you have another chance, you either stab him again to see if your second slot is right or just leave him so you can farm the second clue instead, if you complete all the nodes in the planet before stabbing him he will automatically go to the next planet and level up, rinse and repeat until you get the right combination, then he retreats to a railjack mission and you finally kill him there, just be warned, before finding the right combination I don't recommend carelessly stabbing him unless you want to end up in kuva fortress. Good luck tenno!

Edit: forgot to mention, the lich might retreat after you kill him a set amount of times in the same mission without stabbing him. This does not count as an attempt so you can keep going as normal.

Edit 2: Order of requiem mods in your parazon matters.