If the source folder is in a APFS volume, then by default a APFS volume will be created in the dmg file. This type of volume can not be read by El Capitan. The solution would be to create a dmg file which uses JHFS+ format. The correct command you should be using is given below.
hdiutil create -size <size>b -fs JHFS+ <.dmg-file> -srcfolder <directory> -ov -format UDBZ
Note: When testing this answer, I used the
-megabytes option instead of the
-size option. Also, I tested while running High Sierra instead of Mojave.