Hello SravanKrA,
Thanks for reaching out to the Apple Support Communities! I understand you're having trouble recording a movie in QuickTime where the screen of your iPhone SE or iPad Pro is the video source.
You haven't mentioned any troubleshooting steps that you have tried up to this point. Begin with the following:
1 - Quit all apps that you have open on your Mac. If any will not quit, follow these steps to force it to quit: How to force an app to quit on your Mac.
2 - Restart your Mac: Go to the Apple menu in the top left corner of your screen > Restart.
3 - Restart your iPhone following these steps: Restart your iPhone
4 - Restart your iPad with these steps: Restart your iPad
5 - Verify that you're following the steps under, "Use an iPhone, iPad, or iPod touch connected to your Mac," from this page of the QuickTime Player User Guide: Record a movie in QuickTime Player on Mac.
6 - If the issue persists with the iPhone SE and iPad Pro, create a new user and test it out again. This article will show you how: How to test an issue in another user account on your Mac.
Cheers!