Tasks and tags lock when work is committed or completed, so their dates and details can't change by mistake. A lock icon in the info column of the task grid, or on a tag, shows it's locked. To change a locked item, unlock it first.
Unlock a task
Double-click the task to open Task Properties.
Click the unlock icon at the bottom left of the panel.
You're asked to log a reason as a constraint or an issue. Fill in the required fields and click Submit.
The lock icon disappears from the info column and the panel. Close the panel and edit the task as usual.
Unlock a tag on a board
Open the board and click the locked tag to open it.
Click the unlock button on the tag.
Log the constraint or issue when prompted, then click Submit.
Why you're asked for a reason
Unlocking committed work changes the plan your team agreed to. The constraint or issue you log is saved with the task under its Risks/B&I tab, so everyone can see why it changed.
To lock or unlock the whole task order instead of a single task, see Locking and Reordering the WBS.