error while wiping old mac for recycling

Opened in recovery mode, was able to delete MacHD-data ok, but when tried to delete Mac HD got following error:

"Preparing to erase APFS Volume content. checking mount state, the volume Macintosh HD on disk2s5 couldn't be unmounted because it is in use by process 646(kextcashe).Couldn't unmount disk: (-69888). Operation failed"

What do I do next?