You can't buy or send shoutouts through either the iOS or Android app of pixiv Sketch.
They can be bought and sent using the following platforms:
・pixiv Sketch (on browser)
https://sketch.pixiv.net/lives
・the pixiv app
For iOS: https://apps.apple.com/jp/app/pixiv/id337248563
For Android: https://play.google.com/store/apps/details?id=jp.pxv.android