This could be caused by a few different scenarios. The first possible issue is that you are trying to send funds from a wallet that has not been imported. To find out more about importing, click here.
The second likely issue is that you are trying to sweep your wallet but you are not accounting for network fees, if this is the case, click on the max button when sending and the wallet will calculate the maximum you can send factoring in network fees.
The last possible scenario is that you are simply trying to send more XMR than you are holding. If none of these apply to you, please contact our support team.