Meanwell LED driver usage
You are misinterpreting the datasheet- it means that the input voltage must be at least 3V higher than the output voltage to guarantee current regulation. The output voltage will actually vary with LED voltage to maintain a regulated output current when it is in regulation. I can see how you might take it literally. 

With a significantly insufficient input voltage you will get insufficient output current, maybe not enough to get much light. 

So either give it more input voltage, a more appropriate LED array, or a different LED driver.