@EDUAGILITY - You can lock cards as long as they are outside of Kanban, User Story, and Grids. Here's a card outside of the above objects - the Lock option appears:

And when the card is inside of Kanban, User Story, or Grid object, there is no Lock option:

Additionally, if you are a Guest Editor, you will not be able to lock any objects at all.

Thank you so much for the answer. I have created a calendar using cards and wanted to lock, and could not understand why. Now is clear.

Thank you

Daniel