"Do not tell him anything!"
Translation:Erzählt ihm nichts!
Duo does not accept "Erzähle ihm nichts", which is simply addressing ONE person, instead of many. I have reported this.
Both versions (with/without final -e) are correct, although I agree that you're probably more likely to hear ‘erzähl’.
Its an imperative plural (in german), and imperatives use the imperative form.
The imperative plural form is identical with the indicative plural second person form (you might also call this form the ihr form).