Apple is no longer signing the iOS 17.5.1 update after the release of iOS 17.6.
iPhone users are encouraged to update to the latest version after the signing of iOS 17.5.1 stopped. The next update, iOS 17.6 launched on July 29 and came with a few bug fixes and performance improvements. Apple regularly updates iOS with newer versions to eliminate issues and for security purposes and stops signing older updates in the process. These typically take place several weeks after the new update is made available to the public. When an update is stopped, it means iPhone users can be installed as a server-side check is performed before allowing the process.
With older iOS versions no longer available, users will have to keep their devices up to date, software-wise in order to have the latest security measures in place.
