Before enabling timesheets in your account, make sure you have done the preparation work to ensure a smooth roll-out: Setting Up Timesheets in Your Account
As an Admin user (with Edit Account Settings and Data set as On), hover your cursor over your name in the top right corner
Click Settings
Click Account Settings in the horizontal bar
Click the Edit button on the right-hand side
Scroll down and look for Timesheets
Toggle on Timesheets
Click Save