Update not available my phone why

Go to Settings/General/About and note the iPhone Model Name, then check its version compatibility from this list:

• iPhone 8 or newer, including SE 2nd and 3rd editions, you can update to 16.2
• Any iPhone series 6S and 7, and any model SE: You can update to 15.7.2
• iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.6
• iPhone 5: You can update only to 10.3.4
• iPhone 5C: You can update only to 10.3.3
• iPhone 4S: You can update only to 9.3.6
• iPhone 4: You can update only to 7.1.2

See if your unidentified iPhone model supports more recent iOS versions by taking a look at this compatibility chart: