different banks have different default balance, the higher prestige you are, the more banks you will unlock with each having a higher default balance
this means that all banks' (available to you) balances are too low to be robbed, so you cannot bankrob. to get past this, progress to a higher prestige to unlock more banks
lawyers help you to not lose as much money as you would. when robbing players however, you will lose no money if you fail.