Why i can purchase from any app but i cant purchase from brawl stars?

You may have Screen Time Restrictions enabled for In-App Purchases.

Watch the GIF below for more clarity

Try to delete the app and re-downloading it again from the App Store.

You may need to have a payment method on file, even if the app you want to download is free. 

Check which payment method you have on file and make sure it hasn't expired, been declined or failed.

See the following Apple articles:

If you can’t download or update apps on your iPhone or iPad

If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)

Require a password for App Store and iTunes purchases

Require a password for App Store and iTunes purchases - Apple Support