Why do the less than(<) and more than(>) symbols don't work?

The operators <, <=, =, >=, and > are used to compare your score with another player's score.

execute if score Alice Kills > Bob Kills run tellraw @a {"text":"Alice is winning!"}

In your command, 200 is actually a player name:

execute
  if score XX mana < 200 mana           # If XX's `mana` score is less than `200`'s `mana` score...
run setblock 6582 74 -266 minecraft:air # then set a block.

To test for a certain number, use the matches keyword:

execute
  if score XX mana matches ..200        # If XX's `mana` score is equal to or less than 200...
run setblock 6582 74 -266 minecraft:air # then set a block.

Instead of using comparison operators, use ranges to specify allowed values.

• 0: exactly 0
• ..0: equal to or less than 0
• 0..: equal to or greater than 0
• 0..1: between 0 and 1 inclusive
• 0.1..1.5: between 0.1 and 1.5 inclusive
• ...1: equal to or less than .1. (.1 == 0.1)
• 1...: equal to or greater than 1. (1. == 1.0)