In philosophy, esp. in moral philosophy, the distinction between Seinsaussagen and Sollensaussagen is fundamental. Seinsaussagen are assertions about how the world is (to be is sein in German). Sollensaussagen are statements about how the world shall (shall or ought is sollen in German) be (in moral terms). The distinction is important, esp. for Kant, because it is impossible to infer Sollenssaussagen from Seinsaussagen. Such an inference is called Seins-Sollens-Fehlschluss (in english: is-ought-fallacy or is-ought-problem) or a violation of Hume's Law, after David Hume. This distinction of those two different kinds of assertions is very much part of the core of Kant's moral philosophy.
The english word for Sollenssaussage is moral judgement or normative statement and the english word for Seinsaussage is positive statement.
The given sentence
In den beiden Schriften untersucht Kant die Voraussetzungen und die Möglichkeit moralisch verbindlicher Sollensaussagen.
could be translated into
In both works, Kant is exploring the prerequisites and possibilities of morally binding statements about how the world shall be.
In both works, Kant is exploring the prerequisites and possibilities of morally binding normative statements.