
fehlend
missing
Not present, not there, or not available, although it is expected or should be there.
Er suchte lange nach dem fehlenden Puzzleteil.
He searched for a long time for the missing puzzle piece.
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missing
Not present, not there, or not available, although it is expected or should be there.
Er suchte lange nach dem fehlenden Puzzleteil.
He searched for a long time for the missing puzzle piece.

lacking
Describes the absence of an abstract quality, ability, or feeling; lacking.
Trotz seiner Intelligenz hinderte ihn sein fehlender Mut daran, die Chance zu ergreifen.
Despite his intelligence, his lacking courage prevented him from seizing the opportunity.
The word 'fehlend' (missing) is the present participle of the verb 'fehlen' (to be missing). It functions like an adjective and describes something that is not present at the moment of speaking.
Er ergänzte die fehlenden Daten in der Tabelle.
He added the missing data to the table.
A sentence with the verb 'fehlen' (to be missing) can often be reformulated with the adjective 'fehlend' (missing). For example, 'Eine Seite im Buch fehlt' (A page in the book is missing) becomes 'die fehlende Seite im Buch' (the missing page in the book).
Die fehlende Unterschrift machte den Vertrag ungültig.
The missing signature made the contract invalid.
'Fehlend' (missing) is often combined with things that should be part of a whole. Typical examples are: fehlende Informationen (missing information), fehlende Unterlagen (missing documents), ein fehlendes Teil (a missing part), or eine fehlende Zutat (a missing ingredient).
Bitte reichen Sie die fehlenden Dokumente so schnell wie möglich nach.
Please submit the missing documents as soon as possible.
lacking
For abstract qualities, fehlend and mangelnd can seem similar. Fehlend tends to suggest that the quality is not present at all, whereas mangelnd can also describe a quality that is present but insufficient. So fehlend is the better fit for a complete absence, while mangelnd is usually used for too little.

Wegen völlig fehlender Erfahrung braucht die Bewerberin eine grundlegende Einführung.
Because the applicant has no experience at all, she needs a basic introduction.
Wegen mangelnder Erfahrung braucht die Bewerberin noch etwas Unterstützung.
Because of her limited experience, the applicant still needs some support.