What is the difference with "Wir schreiben die Grüße"? When should one use "erstellen" instead of "schreiben"?
Without further context, I wouldn't use 'erstellen' at all. 'schreiben' is a much better translation. There are some contexts where 'erstellen' would be appropriate, e.g. if you print some greeting cards.
It works when we say "we issued the greetings"