Settings.bundle version number is updating as $(MARKETING_VERSION)

It worked by displaying MARKETING_VERSION itself: Thanks @dgimb and @Mojtaba Hosseini for your answers.

version="$MARKETING_VERSION"
version+=" ("
version+=`/usr/libexec/PlistBuddy -c "Print CFBundleVersion" $SRCROOT/MyApp/Info.plist`
version+=")"

/usr/libexec/PlistBuddy "$SRCROOT/MyApp/Settings.bundle/Root.plist" -c "set PreferenceSpecifiers:1:DefaultValue $version"

The version string $MARKETING_VERSION as well as build number $CURRENT_PROJECT_VERSION are now exposed as environment variable during the build process as they are now persisted in the .pbxproj configuration.

You should be able to achieve what you want like this:

version="$MARKETING_VERSION ($CURRENT_PROJECT_VERSION)"
/usr/libexec/PlistBuddy "$SRCROOT/MyApp/Settings.bundle/Root.plist" -c "set PreferenceSpecifiers:1:DefaultValue $version"