ungenügend
insufficient
Not adequate in quantity, quality, or degree; not meeting the requirements.
Die Beweise waren für eine Verurteilung ungenügend.
The evidence was insufficient for a conviction.

insufficient / failing
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insufficient
Not adequate in quantity, quality, or degree; not meeting the requirements.
Die Beweise waren für eine Verurteilung ungenügend.
The evidence was insufficient for a conviction.

The word is built simply: the prefix 'un-' negates the word 'genügend' (sufficient). 'Ungenügend' (insufficient) therefore literally means 'not sufficient.'
Die Beleuchtung im Raum war für die feine Arbeit ungenügend.
The lighting in the room was insufficient for the delicate work.
'Ungenügend' (insufficiently) can stand not only as an adjective before a noun, but also as an adverb describing a verb. It then means 'in an insufficient way.'
Die Mannschaft war ungenügend auf das wichtige Spiel vorbereitet.
The team was insufficiently prepared for the important game.
In the German school system, 'ungenügend' (failing) is the fixed term for the worst grade, grade 6. In this context, it is not a general description but a defined technical term.
Sein Test wurde mit „ungenügend“ bewertet, weil er keine einzige Frage beantwortet hat.
His test was graded 'ungenügend' (failing) because he did not answer a single question.
When the word is used as a noun, 'ein Ungenügend' (a failing grade), it almost always refers to the school grade 6. For example, you can say 'ein Ungenügend bekommen' (to get a failing grade) or 'im Zeugnis ein Ungenügend haben' (to have a failing grade on one’s report card).
Für diesen Aufsatz hat sie leider ein Ungenügend bekommen.
Unfortunately, she got a failing grade for this essay.
enough
'genug' and 'ungenügend' both relate to whether something is sufficient. But they are opposites: 'genug' means that the amount or degree is sufficient. 'ungenügend' means that something is not sufficient or does not meet the requirements. They can be confused because both are connected to 'genügen' (to be sufficient). A practical tip: If something is sufficient, you say 'genug'. If something is not sufficient or is too poor, you say 'ungenügend'. Also, 'genug' is usually an adverb, while 'ungenügend' is an adjective.

Die Vorbereitung auf den Termin war ungenügend, deshalb mussten wir ihn verschieben.
The preparation for the appointment was insufficient, so we had to postpone it.
Für die Suppe ist noch genug Gemüse im Kühlschrank.
There are still enough vegetables in the fridge for the soup.