$20 is the minimum available balance you need in your bank
account. There's also another balance which is the Ledger balance,
which is not an updated version of balance in your account. Ledger
account balance means the transaction that has been performed
but not yet send through.
Exmaple would be
Available balance: $19.90
Ledger balance: $250.90
In this case, you will not be able to withdraw your money
because it's below $20.
Sometimes, even if your bank have $20.10, your bank may not
allow you to withdraw that sum of money.
