Maybe the phone is out of memory, or it's broken.

Each system will occupy the memory of the mobile phone. You must download the update ios 15 before you can install it. If your mobile phone has insufficient memory space to download, it will not be updated. Either change your mobile phone, or uninstall something first and come back after the next update.

When the system is updated, files need to be downloaded online. If your network is not very stable or good, it will lead to file download problems, and Apple's update system can only use wireless network to update, and will not update your data, so you need to find a stable network area to update and download.