I cannot delete Apps from my App Library in ios 15.

If the app is only in the App Library screen (swipe left past the last Home screen), touch and hold the app there and then tap Delete app.

Note that some built-in Apple apps cannot be deleted.

If the delete option isn't available, there may be a restriction set on deleting apps. On an iPhone or iPad, go to Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Deleting Apps.

See: https://support.apple.com/en-us/HT201304#prevent-purchases