Your external account(s) can only be enabled for Funds Transfer services after it has been verified. Check your Accounts page. If verification of the account is still pending, or if you haven't begun the verification process, you'll see the account listed in your Accounts page under the section titled Accounts Requiring Verification.
When the verification process has been completed, the account will automatically be listed under the section labeled Enabled Accounts on your Overview page and appear on your Transfer Funds page. Then you will be able to schedule transactions from that account.