Unable to link accounts

I'm trying to claim Amazon credits. I created an account at: https://aws.amazon.com/startups/ and then I created an account at https://portal.aws.amazon.com/ and now the first account asks to link the second. However, when doing this I receive the following message: "It is not possible to link accounts. You are not allowed to link accounts. Check your IAM settings or contact the administrator to add relevant IAM permissions." Can anyone help me solve this? Note: I'm using the root user to access the accounts.