At the top of every Transactional Template you'll find a list of variables to use.

Variables check to see who the message is going to and then insert specific info in place of the variable. For example, by inserting {{first_name}} into a message, the recipient's first name will be inserted.

Your variable options are limited to what's listed at the top of each template. Ie. if the variable you're interested in using is not listed above the template you're editing, it won't work if you add this variable to the email's body.

Use exact spelling. To be safe, copy and paste the variable into the email's body.

Ie. here's a potential spelling mishap:

{{first_name}} will insert the customer's first name.

{{firstname}} will just insert the word {{firstname}} and look odd to the customer.