"Above all, don't forget to send the attachment."

Translation: Surtout, n'oublie pas d'envoyer la pièce jointe.

[RubyDenton]

Why is s the d' necessary?

[GraemeSarg]

Because the French expression is:
"oublier de comment/faire/pourquoi" => "to forget how/to do/why".

[ALBALUCIAC2845]

why do not accept " n'oubliez pas " I think it is also correct

[Poef9]

I don't see a mistake. You may want to report if the remainder of the sentence is also correct.

[--Roody--]

Surtout, n'oubliez pas d'envoyer la pièce jointe. was just accepted.