If it is about stealth takedown, don't wait for the second icon to come up, the timing is super tight, just press the first button when the blue icon is showed and then mentally count 3\4 of a second and press the block button, it should work most of the times.
If it is for the lockpicking, yeah it's trash but i can give you 2 hints:
Help the dude who can make yo enter the wedding, not the blacks ith, the other one his quest explain to you how to lockpick, steal and stealth takedown, and you can repeat the training of these things to get levels and perks, lockpick get slightly easier the higher is the level.
When you start lockpicking, you can check where the yellow ball position is, but you can exist and re-enter to change the position of the yellow ball in a more comfy position for you (hard to explain vut try and you are gonna understand).
People started doing sidequest but i think the game wante you to do a couple of main mission where it explain all the systems.