I lost my phone recipt invoice

Did you buy the phone directly from Apple? If yes, and it was within the last 18 months, open the Apple Store (not App Store) app on your iPhone. Tap Products on bottom. Then tap the little circle with your avatar or initials top right. Tap Orders and the devices you've purchased will appear there and you can see the receipt.

If you bought it from someone else, contact whoever you purchased it from.

Contact the place where you got your phone for a new copy.