Delete App from App Library - IOS 14.4 - iPhone SE 1st Gen

Some system apps cannot be deleted,

but if it happens to any app check this article:

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

One of the options you have there is to disable deletion off apps, see if that's what is disabled.

Prevent iTunes & App Store purchases

You can also prevent your child from being able to install or delete apps, make in-app purchases, and more. To prevent iTunes & App Store purchases or downloads:

1. Go to Settings and tap Screen Time. 
2. Tap Content & Privacy Restrictions. If asked, enter your passcode.
3. Tap iTunes & App Store Purchases.
4. Choose a setting and set to Don't Allow.

In your case allow deletion off apps it if is set to Don't Allow.

Good luck!

Turn off Screen Time, delete the app and then turn Screen Time back on.