There are a few things that can cause this.
The first possibility is that you are trying to send funds from a wallet that has not been imported. If this is the case, you will need to contact our support team and send them your wallet address so they can manually import your wallet for you.
The second likely issue is that you are making a transaction and not accounting for the fees. When trying to send your entire wallet balance out, you need to factor in the transaction fees and type in a lesser amount that accounts for the 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.