I'm trying to understand the sequence of steps you're referring to in this question. So you are:
- Selecting to create a new Ticket Template
- Choosing the form for the template
- Attempting to add an Attachment to the Attachments field on the form selected?
At step 3 is the breakdown because the Attachment field isn't showing, right?
Does the form you selected otherwise have that Attachments field included in it, when you view it in the builder in Admin?