My phone is not changing it’s language to punjabi

Punjabi is not a supported system language. However, the iPhone 13 has a Punjabi keyboard you can enable - Add or change keyboards on iPhone - Apple Support

Additionally, you can submit feedback here - Product Feedback - Apple

iPhone 13 Pro

Languages 

• Language support
• English (Australia, Canada, India, Singapore, UK, U.S.), Chinese (Simplified, Traditional, Traditional Hong Kong), French (Canada, France), German, Italian, Japanese, Korean, Spanish (Latin America, Mexico, Spain), Arabic, Catalan, Croatian, Czech, Danish, Dutch, Finnish, Greek, Hebrew, Hindi, Hungarian, Indonesian, Malay, Norwegian, Polish, Portuguese (Brazil, Portugal), Romanian, Russian, Slovak, Swedish, Thai, Turkish, Ukrainian, Vietnamese