That error usually shows up for the older flash games.
Recent versions of browsers require you to allow flash as both a main setting and also a per site basis.
I know that Chrome has a main setting that needs to be enabled and then when you try to play a game it will ask you to allow it on the top-left of the window.
Whatever browser you are using will have something similar.
You likely allowed flash for this site at some point and as long as you don't clear the browser history, it will remember the allowance for this site.