My fusion drive has split.

My fusion drive shows as 2 drives and when I try to repair it through disk utility, I get the following error message: BTW I have installed MacOS on the HDD and left the SSD empty. It runs very slowly.

Kevins-iMac:~ kevinheath$ diskutil resetFusion

Internally-located hardware disk device known to the currently-running macOS:

Rotational                               (disk1)

Volumes exported by partitions or storage systems hosted on the above device:

Flash Storage                            (disk2s1)

iMac HD                                  (disk3s1)

Your computer must have exactly 1 solid-state and 1 rotational disk drive