Wing Yip So

I have registered and received post from HMRC telling me to complete tax return. The post also give me my UTR and confirming they have set up a self assessment record for me. However, when I sign in to https://www.gov.uk/log-in-file-self-assessment-tax-return , I cannot find any button in the page to file tax return. Have I done something wrong or do I have to wait some more time?