Error : -69888: Couldn't unmount disk on my SSD Partition

This iMac originally had a Fusion Drive consisting of a 1TB HDD and a PCIe flash drive? If I'm correct, then all you can install on the flash drive is macOS. If you want, you can leave the SSD as-is, and only use the HDD since the SSD is too small for anything useful. However, you could reconstitute the Fusion Drive.

How to fix a split Fusion Drive - Apple Support

What to do if your Mac’s Fusion Drive fails (or shows signs of failure)

Here is a Solution for Repartitioning a Fusion Drive

Answered by AskDifferent's FROGGARD and edited slightly by Kappy.

Run the following commands in Terminal. Press RETURN after each. The results from the first command are needed for the second command. See explanation below.

   diskutil cs list

   diskutil cs resizeStack LVUUID PVUUID 3.128t

The first UUID is your LV UUID, the second is the PV UUID that you want to grow (incase I mistyped one of them). They will be shown in the output from the first command line.

This will grow your disk back to fill up your 3TB disk. You may have to reduce that number to 3.1279t or something though. 3.128t should work. Change that value if you have a 1TB Fusion Drive to 1.024t. Unfortunately, you can't specify 100% like you can with many commands.

https://apple.stackexchange.com/users/119675/froggard

Froggard

1,191114

acagnoli wrote:

Thanks for your answer,

I tried the solution of reconstituting the FusionDrive, that leads me to the error code i mentioned (i tried it on initial Sierra install). Gonna try the process from Catalina on the HDD now, i hope it will work this time.

Are you booting into Recovery Mode, Internet Recovery Mode, or from a bootable USB installer? You cannot erase or create a Fusion Drive while booted from the drives which will be part of the Fusion Drive.

If you still have a problem, then I found this: How to fix "Couldn’t Unmount Disk" Error on Mac?