The essence of the authority of Scripture is its ability to compel us to believe and to obey it and to make such belief and obedience equivalent to believing and obeying God Himself.
Because God is a God who cannot speak a lie his words can always be trusted. Since all of Scripture is spoken by God, all Scripture must be unlying, just as God is: there can be no untruthfulness in Scripture.
Here the author says not merely that God does not lie, but that it is not possible for him to lie.