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.

missing
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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.

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.
available / present
'vorhanden' and 'fehlend' are clear opposites. 'vorhanden' means that something is there or available. 'fehlend' means that something is not there, even though it is expected or needed. Both words are often used in factual situations, for example with documents, parts, information, or materials. So the important difference is: 'vorhanden' describes something being there, while 'fehlend' describes a gap or something missing.

Die fehlenden Seiten werde ich morgen noch kopieren.
I will copy the missing pages tomorrow.
Alle nötigen Daten sind vorhanden, sodass wir den Antrag abschicken können.
All the necessary data is available, so we can send off the application.