The iOS version of the pixiv Sketch app does not allow users to purchase or send Shoutouts.
(That is because there is no Live viewing feature in the pixiv Sketch app.)
If you want to purchase or send Shoutouts, you can do so from one of the means below.
・ pixiv Sketch (browser)
・ pixiv app
iOS version: https://apps.apple.com/app/pixiv/id337248563
Android version: https://play.google.com/store/apps/details?id=en.pxv.android&hl=en