unvollständig
incomplete
Not complete; parts, information, or elements are missing.
Bitte füllen Sie das Formular aus, es ist noch unvollständig.
Please fill out the form; it is still incomplete.

incomplete
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incomplete
Not complete; parts, information, or elements are missing.
Bitte füllen Sie das Formular aus, es ist noch unvollständig.
Please fill out the form; it is still incomplete.

The word is clearly structured: the prefix 'un-' negates the adjective 'vollständig' (complete). 'Vollständig' means 'complete', so 'unvollständig' logically means 'not complete'.
Deine Antwort ist leider unvollständig.
Unfortunately, your answer is incomplete.
This adjective is often used to describe documents, information, or collections. Typical combinations are 'unvollständige Angaben' (incomplete information), 'unvollständige Unterlagen' (incomplete documents), or 'eine unvollständige Liste' (an incomplete list).
Wegen unvollständiger Unterlagen konnte der Antrag nicht bearbeitet werden.
Because of incomplete documents, the application could not be processed.
'Unvollständig' often appears as a predicate adjective after verbs like 'sein' (to be), 'bleiben' (to remain), or 'wirken' (to seem). In this position, it is not declined.
Ohne deine Unterschrift ist der Vertrag unvollständig.
Without your signature, the contract is incomplete.
partly
'teilweise' and 'unvollständig' both have to do with something not being complete. 'teilweise' is an adverb and describes how or to what extent something applies: only in part. 'unvollständig' is an adjective and describes the thing itself: parts or information are missing. They can be confused because both stand in contrast to 'vollständig' (complete). As a practical guide: 'teilweise' often fits with verbs or whole statements, while 'unvollständig' fits with documents, lists, details, or other things that are missing something.

Für die Anmeldung ist das Formular noch unvollständig.
The form is still incomplete for the registration.
Beim Termin konnte der Mitarbeiter die Fragen nur teilweise beantworten.
At the appointment, the employee could answer the questions only partly.