Writing/shorting of listed options requires activation of an advanced options trading profile. This is only applicable for stock options and options on futures & indices.
In order to enable it, you will have to acknowledge that you are aware of the risk involved in shorting options and have relevant experience by reading and accepting the conditions.
To request the advanced options trading profile, please submit a ticket, by going the main menu 
> Your account > Request account services > Enable Short-Selling of Listed Options > confirm you have read and accepted the conditions and Submit.