It would be Really Good if the Error message listed all the available Backend names.
Better error msg:
ERROR: no backend specified. Please use 1 of: html5 | hxwidgets | openfl | nme | pixijs | kha | winforms | android | flixel | electron | qt
Spent some time trying to get heaps to work and then saw Heaps was not in supported list of haxeui-core Backends.