If you are running into this scenario, it is because you have 'Unsettled' deposit transactions.
Basically you have deposits that have not fully cleared (7 business days). You can review the uncleared transactions by clicking 'See ACH Transaction History' and inspecting any transaction that is in POSTED status. Once these move to SETTLED status, you will be able to withdraw them.
Refer to this article for information on our 'Instant Deposit' process