when i create my account , i will 20$ free credit but i have receive nothing in my account

Where did you read that? Are you sure it was an official Linode page, not some referral spam website?

i use the official linode page from search,and use the new promote code "SYDNEY20" .it is my only one ,and new account ,hope you can check my account.yesterday i once a time see the 20$ free credit . thanks