There are two setting options for displaying the assignment 'Description' in the course room.

General > Show description on the main screen

The description you wrote appears under the Assignment Module in the Course Home.

(Note) It is not visible if it is set to 'Always show comments - unchecked' before the starting date/time.

Setting Submission Duration> Always Show Description

The description you wrote appears at the top of the assignment page.

When unchecked, it is displayed after the starting date/time.

(Note) The 'Always show description' option can be set after setting the starting date/time. A description will be always displayed if the starting date/time are not set.