Try all of these below, note that while expansion upgrades are quick the boost itself may take up to 24 hours to appear when the payment is being checked out.
- Visit your Transaction History and verify you purchased the Heroic or Epic edition. The Base edition does not include a boost. Also verify the order status is Complete.
- If you have multiple World of Warcraft accounts on your Blizzard account, make sure you launch the account you upgraded to Shadowlands. You can only use the free character boost on the WoW account you upgraded.
- Create a level 1 character to refresh your character selection screen.
- Log into a character, then log out back to your character selection screen.