i cannot buy in app purchases and it show :you cannot make purchases on this device yet:

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

See also 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

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

Watch the GIF below for more clarity