Failure to mount .dmg or view external drives!

I completely agree with @Luis Sequeira1 regarding CMM. Do not install CMM as it definitely tends to cause all sorts of problems for users sooner or later. In fact, do not install any anti-virus apps, cleaning/optimizer apps, or third party security software since these types of apps are not needed on a Mac and they usually cause more problems than they solve. You should review this excellent article written by a respected forum contributor on how to keep your computer running smoothly & keeping you & your computer safe:

Effective defenses against malware and other threats - Apple Community

In order to see whether you have any other third party software installed which may be interfering with the normal operation of macOS, please run EtreCheck and post the report here so we can examine the report looking for any clues.

Is the only issue you are reporting is not being able to open the CMM .dmg file? Or is it some other .dmg file you cannot open?

Make sure to purchase at least one external drive (at least 2 or 3 times the size of your internal drive ... other contributors can correct me if I'm wrong) to be used for your backups. macOS includes the Time Machine backup software for free with macOS so users can easily backup their computer.

I am now running Ventura but I mounted many dmg when using Monterey, and continue to do so in the current OS.

There is something wrong with your system installation, not Monterey in general. And installing a program that earned the deserved nickname 'brickmymac' is not going to help, anyway.

If you want to troubleshoot the real issues, you can run Etrecheck and post the report so others can help.

If not, then it's your mac, your call.

Cleanmymac is absolute GARBAGE.

No backup? I am sorry if you lost documents, but that, I have to say, is on you. If you did not make a backup going back 15 years, you did not value your data that much. Sorry to be blunt, but it is true.

There is no bug regarding dmg files. If your mac has a functioning OS and it cannot open it, it more likely means that said dmg is corrupted.

I have no idea what you have done to your mac, or why you had to restore in the first place.

Maybe there is a hardware problem?

If you give us more information perhaps we can help.

In situations like this, it is always best to explain what you are seeing. Ideally provide a screenshot.

Otherwise, people have no idea what you are talking about.