Not able to use stickers since buying Iphone 16 pro max

OK....Try a Forced Restart on your iPhone.

Follow this precisely:

• Press the volume up button quickly and release
• Press the volume down button quickly and release
• Then Press and KEEP HOLDING the side power button and KEEP HOLDING it past the swipe to power and KEEP HOLDING after the screen goes dark and KEEP HOLDING until you see the Apple Logo, then LET GO

Once your phone restarts, sign back in and see if you can use stickers normally

It's not intuitive. You don't go to photos to make a photo available as a sticky.

Open the message to start typing. Tap the + next to the dialog box. Tap Stickers. Tap the icon on the upper left The one which looks like a circle with a portion out of it, which then lets you choose a photo you want to turn into a sticker. It will outline the object in the photo, then you can use that Sticker in the message. I just made a Sticker this way and it worked as expected.

Sorry. I don't have a clue why it isn't working for you. It works flawlessly on our 16 Pro and 16 Pro Max. I'd recommend contacting Apple Support for help. If you're in the US, you can call them at 800-MY-APPLE. You can also schedule an appointment at your Apple Store Genius Bar and show it to Apple in person.

See this for guidance on Stickers --> Send stickers in Messages on iPhone - Apple Support

Did you click Add Sticker at the bottom right after it highlighted the object? That should add the sticker to your stickers library, and can be used anytime you want.