Why my mobile cannot update..??

An iPhone 5S can only run as high as iOS 12.5.5 Any iPhone that can't run iOS 15 can't run higher than iOS 12.5.5

iOS 15 compatible devices --> https://support.apple.com/HT212712

Because it is not compatible with iOS 13 or higher.

Hello,

The latest version of iOS compatible with your iPhone 5S is iOS 12.5.5 which was released recently. Are you currently on this version? You can check by going to settings > general > software update or settings > general > about.