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You may have to add a card to verify your account and act as a back up funding source before you can use funds in your balance.
However try this...
Click on the profile tab (icon next to log out) > Click on payments third option along on the thick blue band top of page > second down click on update next to your preferred way to pay and put a dot in the balance option.
Also if you are trying to use funds in your balance then they have to be in the correct currency as paypal do not 'convert' funds within a balance unless you do that yourself. You would also have to go to manage currencies and make that currency your primary one for that transaction.