Chocolate mint ganache - too runny

I think you've got too much liquid in your ganache, either because the chocolate or the glucose syrup (assuming its syrup). Milk chocolate can be used in a ganache but as it has a lower proportion of cocoa you would want to use less cream in your recipe when using it.

You could try whipping it as the inclusion of air should thicken it, however that will change the consistency so it won't be smooth and glossy. I would suggest you reheat it and melt in another 30 grams of good quality dark chocolate or so. That amount is not exact or based on any sort of science on my part, more instinct from previous ganache making experience. You might need to add more depending on your circumstances.

When your ganache is runny, it is usually a matter of proportion.

Typical ratios of pure (chocolate/cream) ganaches run between 1/1 and 2/1 if you want them non-runny, and 1/1 can be quite soft. When you consider that cream has 60% liquid and 40% solids, that's a ratio of between 2.33/1 and 4/1 pure solids to pure liquids. Your combination, if we don't count the glucose, has a ratio of 225/85=2.64/1, which is very much on the soft side. So the first thing would be to use more chocolate.

Second, ganache assumes pure chocolate. Not necessarily couverture quality, but it should be nothing more than chocolate raw mass, chocolate butter, and sugar. Random chocolate bars from the supermarket tend to have too much sugar, vegetable fats instead of cocoa butter, milk solids, and sometimes also some specific emulgators and other texture-changing agents. So you can use a supermarket bar, but it has to be one with the proper ingredients - Lindt Excellence (not Lindor!) tends to be a globally available brand, but there are other options too, just read the ingredient list. And make sure to use the cocoa percentage required by the recipe. If nothing is mentioned, assume it is pure chocolate - if that is too expensive, you can substitute one with 85% or 90% cocoa content.

Then there is the glucose. I don't know why you are using it for piping ganache - it is added when you need plastic chocolate for sculpting by hand. I have not worked much with it, so cannot say if you need to start out with a thicker base ganache to compensate for the liquid in the syrup, but it is a possible reason why it went wrong.

So, in conclusion, I suggest that you adjust the ratios and ditch the glucose. And use proper chocolate. Try maybe 200 g chocolate, 115 g cream, 10 g mint essence (to approximate a 3/2 ratio) and see how it goes. Then you can adjust the next batch with more or less chocolate.