Refund

You can follow the steps from the following support article to request a refund for a purchase you made through an app: Request a refund for apps or content that you bought from Apple - Apple Support

I’m nor sure if Apple will refund a TikTok purchase, but you can try this:

How to request a refund

1. Sign in to reportaproblem.apple.com.
2. Tap or click "I'd like to," then choose "Request a refund." 
3. Choose the reason why you want a refund, then choose Next.
4. Choose the app, subscription, or other item, then choose Submit. If you were charged for a subscription that you no longer want, you can also cancel the subscription.

Steps were copied from this link:

Request a refund for apps or content that you bought from Apple - Apple Support

If you paid via Apple Pay, then that’s a debit or credit card purchase, and between you and TikTok and your payment card provider.

If you paid via a subscription or by an in-app purchase that used a payment method associated with your Apple ID, see the replies above.