Under "panel preferences", "appearance", look at the option called "Transparency": when it's set to 100 (maximum transparency) the panel has no transparency (100% opacity); when it's set to 0 (minimum transparency) the panel has maximum transparency (0% opacity).

This is easily fixed: either rename the slider to "alpha" or "opacity" or something that accurately indicates what's being adjusted; or else (if it remains a "transparency" slider) reverse the scale so that 100 does in fact produce 100% transparency (rather than 0% transparency as at present).