What are you playing it on? The game is for PC so it might not work flawless on other devices. It looks like it can't set the opacity of some image that doesn't exit but it thinks it should ("null"). Don't know how to help with that.
You should take a look to see if the slime enemies are in the www/img/enemies folder. maybe it can't read those for some reason.
If you're using Joiplay try to see if you have set it correctly to play rpg maker games, there should be tutorials on youtube. I think there was a plug in you had to download for it, but I can't remember it's been a while.