Can't reformat disk from failed RAID array in Disk Utility

Choosing "Delete RAID..." gives:

An internal state error occurred

Operation failed…

It was a stripe of an SSD with a stripe of two HDDs. The SSD got 50%, the other two, 25% each. Then I noticed one HDD clicking and failing, removed it. Now I'm stuck I want to erase the other two disks but can't!

Have rebooted and still same issue. Cant choose "Erase Disk" as the option is greyed out!

First aid is always "ok fine".

Try to

Remove “disk3s2” from RAID set “Untitled”?

Removing member “disk3s2” from the RAID set will attempt to preserve data if possible; otherwise, this will delete all data stored on it, and cannot be undone.

remove failed...

An internal state error occurred

Operation failed…

10.12.6 (16G1510)

MacBook Pro (Retina, 13-inch, Mid 2014)