when I’m going to install ios 15.7.1 i get install failed error

Greetings Mrzook98,

If you are unable to update iOS on your iPhone, follow the guidance found here:

If your iPhone or iPad won't update - Apple Support

"If you can't reach the update server or can't verify the update

If you try to update your device, you might see one of these messages:

"Unable to Check for Update. An error occurred while checking for a software update."

"Unable to Install Update. [Software] update requires a network connection."

Try updating your device again using the same network. If you still see one of these messages, try updating your device using another network or update your device with your computer:

• Update your device using a Mac with macOS Catalina or later
• Update your device using a Mac with macOS Mojave or earlier, or a Windows PC

If the issue happens again after trying to update with multiple networks, remove the update."

Take care.