There are a few reasons why you cannot erase certain partitions: protected system partitions, file system compatibility and disk encryption or file system corruption.
To successfully erase them, you may use specialized third-party partition management software that can handle both macOS and Windows partitions. Tools like EaseUS Partition Master, or GParted can provide more options to deal with complex partition setups.
Before making any changes, ensure you have a backup of your important data. It's worth mentioning that for a fresh macOS install, you don't necessarily have to erase the Fusion Drive and create one partition manually.
As the questioner asked, there are simple guides to erase Fusion Drive and install macOS:
- Once booted from the installer, go to the menu bar and choose Utilities > Disk Utility. In Disk Utility, you should see your Fusion Drive listed. It should appear as one logical volume representing the entire capacity of the Fusion Drive.
- Select the Fusion Drive (the top-level entry, not the individual partitions under it).
- Click the Erase button, choose the appropriate format (usually APFS for modern macOS versions), and give it a name.
- Close Disk Utility and proceed with installing macOS on the newly formatted/erased Fusion Drive.
- Follow the on-screen instructions to complete the installation.
You can refer to[Fixed] Disk Utility Won't Let You Erase Hard Drives
And also Erase process has failed' on Mac Internal & External Drive
