There are different account types for The Trade Desk. You will need to use an account that is specific for API access when connecting to ChannelMix. Usernames of API specific account will start with ttd_api_*. If you have not been provided such credentials, you will want to reach out to your TTD rep to have them provide this for you. You will also want to make sure that the report(s) you want to use are shared with this API credential.
Be prepared to provide us the Partner ID and Advertiser IDs for the ChannelMix Profile you are requesting to be setup.
For information on how to configure your Trade Desk reports, please refer to the article: The Trade Desk | ChannelMix Configuration.