Why can’t I delete an app from my App Library?

Note: Some Apple builtin apps cannot be deleted.

But all 3rd party apps can be deleted, if you do not have restrictions set.

Check Settings > Screen Time > Content & Privacy Restrictions > iTunes and App Store Purchases > make sure Deleting Apps is Allowed.

Also note: Content & Privacy Restrictions must be enabled to see and make changes.

See this and the section titled Delete an app from the App Library. Organize the Home Screen and App Library on your iPhone - Apple Support If you do not have the ability to delete one from there, then go to Settings>Screen Time, Restrictions and turn on the ability to delete apps.

THCreativity Said:

"Why can’t I delete an app from my App Library?"

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Hide this App:

You cannot delete a purchased app forever. All you can do is hide the app.  Go Here: Hide Purchases from the App Store - Apple Support. The only way it would be deleted forever would be if the Developer decides to discontinue licensing it with Apple.