If you wish to offer your customers a product where they can only place a text with no other options, you have two ways to make it:

1) Create a Printing Method where you enable the text tool only (see image below) ;

2) Create a pre-designed template on the product. Go with this option if you wish to set more restrictions on how your customers can use the text tool, such as prevent the user from changing the position and the size of the text, change color, set a limit for the number of characters, force uppercase, and so on.

In the video below we're setting a t-shirt where the client can only add a text and they cannot:

- change font and color

- move, resize or rotate the text

- use lowercase letters

- enter more than 9 characters