Insert variable not showing on 'To' field

Insert variable not showing on 'To' field

Hello, 

I want to create email triggers to be sent out to respondents of our survey as soon as they complete it. I also want a variety of email triggers to go out depending on the answers the respondents submit. We do not however have an 'Insert Variable' option on our To fields. 

How can I sort this so I can send the variety of email triggers?

Kind Regards,

Chris