The following disk image couldn’t be opened error message is displayed.
You may be trying to open the KaleidaGraph v5 for Mac disk image on an unsupported OS. Version 5 requires Mac OS 10.14 or later. OS 10.13 can be used, but there is an extra step involved. However there isn’t any way to use v5 on anything older than OS 10.13 due to the requirements of the underlying toolkit we use.