My refund is 0. Why do I have to pay?

See if you can downgrade to Free edition.   You pay based on the product you are using, not by the amount of your refund.   

Review your fees and remove anything you don't need.   How to Review your Fees

You also have the option to switch to a cheaper TurboTax Online product. Before switching, make sure the new product fits your tax situation and forms.

As a last resort, you can clear and start over to remove all fees and start from scratch as long as you haven't paid, registered, or deducted the TurboTax fee from your refund.   See this TurboTax FAQ

Your fee is not based on the amount of your refund or even on whether you are getting a refund.   It is based on which version of the software you used, and you used a paid version instead of the Free Edition.   You would owe fees for that software even if you also owe the IRS and get no refund.

Use this IRS site for other ways to file for free.  There are 8  free software versions available from the IRS Free File site

https://apps.irs.gov/app/freeFile/

Or—if you live in certain states you may be eligible to use the new IRS Direct File

In Filing Season 2025, Direct File will be available in Alaska, Arizona, California, Connecticut, Florida, Idaho, Kansas, Maine, Maryland, Massachusetts, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Oregon, Pennsylvania, South Dakota, Tennessee, Texas, Washington, Wisconsin, and Wyoming. 

https://directfile.irs.gov/

https://home.treasury.gov/news/press-releases/jy2629