With this Power-Up, your task can be marked as blocked. You can do this from the context menu - right-click on a task and choose the Block task option.
The Power-Up needs enabling per each of your boards, like any Power-Up. Please do that from Settings - Power-Ups - Card Block.
Blocked cards cannot be moved, they have a distinctive look and optionally display a message, explaining the block reason.
So that in case of problems with progressing work on a given task, the card can be shown as blocked, until the issue is resolved. This gives the team an instant clue on that card’s status.