If you ENABLE FLOAT (Settings > Register Settings) at register level, then you will need to OPEN REGISTER before you can enter transactions.
To Open the Register, go to Sales > Cash Control and click OPEN REGISTER button on top-right
Enter the Opening Float amount (starting cash) and click Open Register button below.
To close the register at the end of the day, go to Sales > Cash Control and click Close Register
Enter the amount of cash on hand and then Close Register.