Got a 12v 30a PSU but minimum output is 21v
Looks to me like something is broken- probably something in the feedback section (low voltage side to the optoisolator, or possibly just on the other side of the optoisolator). Contrary to other opinions, my experience is that these types of power supplies (unlike PC power supplies and some DC-DC converters) should work fine down to zero load. Here, for example, is a so-called 'user manual' from a similar product that specifies it will work down to 0A. 

It  could be as simple as a bad solder joint (open connection), so you could look for that. 

That said, my initial impression is that this thing is pretty crappy and I would never connect it to anything important or where isolation failure might cause risk to human life or property, or where EMI was a factor. With all that metal it's unlikely to directly cause a fire but could smoke a lot. I doubt the 'FCC' and 'ISO' stickers refer to any real testing or processes- there are no file numbers and no manufacturer legal name that I can see. There is obviously no overvoltage protection (OVP) on the output or it would have shut down- that would be considered a mandatory feature in most cases. 

Also, note that they did not bother to install the heatsink HS1 on the diode bridge. Probably works fine at 230VAC in China, but may burn up on a North American power supply at full current (especially in high ambient).