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I'm reading the introduction to Kant's Grundlegung der Metaphysik der Sitten and came across this line

In den beiden Schriften untersucht Kant die Voraussetzungen und die Möglichkeit moralisch verbindlicher Sollensaussagen.

The problem is that I can't seem to find a definition for this word anywhere, as if it didn't even exist.

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.

or into

In both works, Kant is exploring the prerequisites and possibilities of morally binding normative statements.

German is full of noun compounds that are not listed in dictionaries. The meaning of such compounds is hopefully derivable from the meaning of the parts. Let's see whether it works in this case.

Sollen: (noun derived from the verb by conversion) roughly obligation, duty
Aussage: statement
Sollensaussage: statement about obligation or duty

_{Note: I wanted to show what I think is a sound strategy when encountering unfamiliar compounds. As Jonathan Scholbach's answer shows, technical terms often have a meaning that cannot be derived (although the derived meaning actually provides a solid basis for understanding the technical meaning in this case). A humorous example would be Spannung (suspense, excitement, tension), which in Physics means voltage. This is the reason why there are specialised dictionaries.}