macOS 26.4 Developer Beta 1 seems to break fsck_hfs. This is discussed here, here, and here, and I have filed bug FB21986959 with Apple. I encourage you to file your own feedback if you’re seeing this on your Mac. The symptoms are that it may not be possible to create disk images and that even disk images created on previous versions of macOS may not be mountable. macOS will report errors like “no mountable file systems.” (This also affects physical storage devices, not just disk images.)
By default, DropDMG uses the HFS+ file system for maximum compatibility. You should be able to avoid this bug by clicking here to tell DropDMG to use the APFS file system instead. Disk images created in this way will not be compatible with macOS 10.12 or earlier. You can click here to go back to the default file system.
If you have an HFS+ disk image that you need to mount under macOS 26.4, it may help to tell DropDMG to convert it (even to the same .dmg format, even without using the APFS setting above, but just to remove the license agreement) or to mount it in Terminal using the hdiutil attach command, which will say something like:
There may be a problem with this disk image. Are you sure you want to open it?
[Opening this disk image may make your computer less secure or cause other problems.]
but allow you mount it if you agree to that.