If you see the Insufficient funds error message when you try to send a transaction, it means you are trying to send more funds than you have in your account (including when accounting for transaction fees). It may appear that you have enough funds to spend, but this is before calculating the fee paid to bitcoin miners to have your transaction included on the blockchain.
Try lowering the amount you are spending (or send more funds to your wallet) to make sure there is enough left over to pay for fees.
If you would like to send the maximum amount possible, select the Send All Funds button - which will automatically adjust your sending amount to account for the transaction fees that will be paid.
Note: If you are sending assets using Liquid, then transaction fees will be paid in L-BTC - so make sure you have enough in your wallet to avoid the Insufficient funds error.
