Well, you don't "open" dmg files. You import them. Is that what you are doing?
I double clicked on the downloaded dmg file.
Normally the installation screen appears to walk you through the process when you control-click on the download package or on Setup.
If you purchased from Adobe you may need to log-in to your Adobe account and download again.
I have downloaded twice with the same result: the dmg file will not open.
I think you need to contact Adobe directly using the link below. Use the dropdown menu for boxes (1) & (2) to scroll down the list and choose:
1. Adobe Lightroom
2. Download, installing and setting up; then downloading and installing
3. Click on the blue button: Still need help? Contact us – then click the button marked “chat now”
It’s usually possible to start a live chat, if an Adobe agent is free, and often to get the problem fixed right away.
Click here to get help now Contact Customer Care
my mistake. I was thinking dng instead of dmg