This error message means that you do not have enough funds in the sub-wallet you selected to send from to cover both the amount you want to send and the recommended fee for your transaction. You can view the size of the recommended fee at the bottom of the send screen and add it to the amount you want to send to check that it exceeds the balance of the sub-wallet selected in the From: dropdown menu.
To continue sending, you have a few options. If possible, choose another sub-wallet to send from that has a greater balance. For sending bitcoin, you can use the Customize Fee option to set a lower custom fee. Please be advised that setting too low a fee will increase confirmation time and possibly cause your transaction to be rejected. You can also lower the amount you want to send to accommodate the fee.
Your transaction fee is set by the Blockchain wallet. This article explains how fees are determined.
If you believe this error message may be incorrect and can see that the recommended fee plus the amount you want to send does not exceed your available balance on your chosen sub-wallet, contact our support team. Please include a screenshot (you can attach it to your ticket in either JPEG or PNG format) of the send screen where the error appears.