I have a iPhone 6 I cannot update higher than a 12.5.7 why?

The non-technical answer is that the iPhone 6 is too old and it lacks the horsepower needed to run higher iOS versions. The product was designed almost 10 years ago using technology that was available at the time.

A bit more technical.....the hardware in the Phone 6 is not capable of meeting the software demands of higher iOS versions.

To run iOS 16.....the most recent iOS version.....you will need an iPhone 8 or higher.

iOS 13 and above require at least 2GB of RAM; nobody on these boards will know if there's going to be another update to 12.

(238912)

Correct; any future updates to 12 won't affect application compatibility.

(238981)