In this recipe, what is a possible substitute for serrano pepper?
The closest thing that's at all common in grocery stores in my experience is just a jalapeño. There might be some alternatives, but I think they'd be even harder to find than serranos. (This is based on my experience in the US, but I'm guessing it'll be true in Europe too.)

Failing that, just use green bell pepper. The flavor is similar, minus the heat, and you can make up the heat with whatever you want, like cayenne or another mostly-just-hot ground chili.