Unable to use Apple ID for TikTok purchases

“… Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase … “

Source:

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

See also:

Payment methods that you can use with your Apple Account - Apple Support

You do not provide us with any information about why you cannot. If you see an error message, we need to know what it says if you wish any real assistance.