Blockstream Green displays an insufficient funds error when you try to spend more funds than are available in your account. The total available balance is displayed in the top section of your account screen.
If you have insufficient funds in your account, you need to deposit more funds or lower the transaction amount in order to initiate a transaction.
Note: If you have a sufficient amount of funds in your wallet, but they are spread across multiple accounts, you may still receive an insufficient funds error. Funds in each accounts in your wallet are kept completely separate. If you need to make a payment in one transaction, you can instead manually send funds from one account to another to reach the required balance.
Blockstream Green always adds a Bitcoin transaction fee on top of each transaction.
This means that you need to make sure that the amount you are sending plus the transaction fee is less than your total balance. Especially at times when there is high traffic on the bitcoin network, and you have Fast network fee selected, you may wish to double-check how much is being deducted for a miner's fee.
Because of this, trying to empty your account by entering the total amount manually will result in an insufficient funds error.
To send all the funds in your account, do not enter the total amount manually. Instead, after going to the Send page, tap Send All Funds. This will send all funds in your account, minus the transaction fee.