Sequoia Install Stuck - OSISDownloadOperation: No IAL update found

Below can be used to do a clean install on Apple Silicon Computer

Just adjust some of the wording were it may apply re-downgrade

On an Apple Silicon Mac, downgrading to an older macOS version requires a full erase of your computer's internal drive. 

You cannot simply install an older version over a newer one. 

Before you begin, you must first create a complete backup of all your data. 

Step 1: Back up your Mac 

This is the most critical step. Downgrading will erase your Mac's internal drive, so you must back up your files and settings beforehand. 

• Time Machine (recommended): Use Apple's built-in Time Machine to create a full backup on an external storage drive.
• Manual copy: Alternatively, you can manually copy all your essential files to an external hard drive. 

Step 2: Create a bootable installer for macOS 15 

To install the older operating system, you will need a bootable USB installer. 

1. Download the installer: On a Mac compatible with macOS 15, download the "Install macOS Sequoia" app from the Mac App Store.
2. Use Terminal: Connect a USB flash drive with at least 15GB of free space. Open the Terminal app (in Applications/Utilities) and use the createinstallmedia command to turn the USB drive into a bootable installer. The command for macOS 15 Sequoia is:
3. sudo /Applications/Install\ macOS\ Sequoia.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
4. (Replace MyVolume with the name of your USB drive)

Step 3: Erase your Mac's internal drive 

1. Shut down your Mac: Turn off your computer completely.
2. Enter Recovery Mode: Press and hold the power button until you see the "Loading startup options" screen.
3. Choose Options: Click the Options icon, then click Continue.
4. Open Disk Utility: From the Recovery screen, select Disk Utility and click Continue.
5. Erase the drive: In the sidebar, select your Mac's internal drive, usually named "Macintosh HD." Click the Erase button and enter a new name for the volume. Ensure the format is set to APFS, then click Erase Volume Group. 

Step 4: Install macOS 15 from the bootable installer 

1. Quit Disk Utility: After the erase is complete, close Disk Utility to return to the Recovery menu.
2. Select the installer: Click the Install macOS Sequoia option. You may need to connect to Wi-Fi at this point.
3. Choose the drive: Follow the on-screen instructions and select the internal drive you just erased as the destination for the installation.
4. Complete the installation: Wait for the installation to finish. Your Mac will restart several times. 

Step 5: Restore your data 

1.  Restore manually: If you copied your files manually, you can skip Migration Assistant and drag your backed-up files back into their respective folders. 

Important considerations 

• Data loss: Any new files or changes made on macOS 25 since your last backup will be permanently lost during the downgrade.
• Application compatibility: Some apps that were updated for macOS 25 may not work correctly on macOS 15.
• Hardware compatibility: Your Mac must be compatible with macOS 15. A Mac that shipped with macOS 26 cannot be downgraded to a version it never supported.
• Firmware: Downgrading to a much older macOS version may also involve a firmware downgrade, which adds risk. The Apple Configurator tool can be used for more robust downgrades.

Let us first establish.

1 - Do you what to do a Clean Install of macOS 26 Tahoe ?

2 - Do you what to do a Clean Install of macOS 15 Sequoia ?

Normally, once macOS 26 Tahoe has been installed

Using Erase all Content and Setting

>>>>>> Use the Erase All Content and Settings feature to quickly and securely erase all settings, data, and apps, while maintaining the operating system currently installed. <<<<<

In effect, there is nothing more to be done