How to make a "execute if score is equal to or greater then x" command

From the Minecraft Wiki:
"@e[scores={myscore=5..}] — Select all entities with a score in objective myscore of five or greater."
In your situation you will need at least two command blocks where the first will be a repeating command block detecting the entities with score higher or equal to 25 and removing the required points:

execute as @e[scores={scoreboard=25..}] run scoreboard players remove @s scoreboard 25

And the other command block has to be a conditional chain command block (has to be conditional so it only executes if the first one is executed successfully) and it is going to unlock the thing.
If you need to do some actions on the player that reached the score you will most likely need more command blocks and will need to use the target selector in each of the command blocks and remove the required score after all the commands targeting the player are done. Alternatively you could use another scoreboard or a team to identify the player.