cannot reset Fusion Dive in Imac 2019 21.5in Mojave

It may be better to copy/paste the Terminal commands from this Apple article so you know there are no typos or case issues.

How to fix a split Fusion Drive - Apple Support

Example: note that Terminal commands are case sensitive. the command is resetFusion, not ResetFusion.

Hi there!

In macOS, please upload a screenshot of your Mac’s storage information. In the Finder, choose Go > Utilities, then double-click the Disk Utility icon. (Disk Utility is located in the /Applications/Utilities folder.) Alternatively, click Launchpad in the Dock, type Disk Utility in the search field, then click the Disk Utility icon. Once Disk Utility is open, take a screenshot of the information it automatically opens to, which will be your startup disk. Next, select View > Show All Devices in Disk Utility and select the top-level, non-indented, disk entry. In most cases, this name will be one of the following:

• APPLE HDD…
• APPLE SSD…
• Fusion Drive

Then, take a screenshot of the newly selected entry. Upload all screenshots here.

Jack

FYI, to reset the Fusion Drive will destroy all data on both the SSD and Hard Drive. If you don't already have a backup, then make a backup now. Then follow the instructions in the Apple article @den.thed provided which will require you to boot into Recovery Mode or from a bootable macOS USB installer.

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

Edit: Where did you encounter the problem in those Apple instructions? We cannot help if we don't know what did not work and the error message received.

Does anything @ > How to fix a split Fusion Drive - Apple Support help?