Please note, that Time Tracking & Reporting are only available on accounts using the Enterprise service plan. To see what plan your account is on and to change it, please go to Account Administration - Plan & Billing details.
Time Tracking will get activated for any task, that is moved to an in progress type of column. It can also be manually activated, under right-click on a closed task - Start timer.
To make sure, that your board’s In progress or Doing column is in fact of an in progress type, go to the board Settings - Board Editor and click the pencil icon under your working column. In there, you’ll see what Lane Type has been set for the column.
You can read more on Time Tracking configuration here.