Trying to create a Mojave VM on my Big Sur mac. The dialog box gives you the opportunity to give the DMG file a different name. But if you rename the DMG file to anything EXCEPT "macOS image file.dmg" the installation fails.
ok now I'm not sure it has to do with renaming -- but it's definitely a bug. I was trying to rename the dmg to "10-14 Mojave.dmg". I did it three times in a row, exactly the same each time. Failed the first two, worked the second. Then I did all that AGAIN (ie, failed another two times, worked the third for no apparent reason.)