First, this is one of those highly subjective things where everyone has a different opinion. So any answers I give here are far from universal. On the whole, I’d say I’m fine with debut authors pitching a series. I know plenty of debut authors who have successfully published series.
A cautionary note, though. When querying, focus only on the book you currently pitching (which I assume would be the first book in the series). I’ve seen authors write query letters where they give the synopsis for the book they’re pitching and then devote an entire paragraph (sometimes more!) to talking about the plot for the sequel. I don’t want that!
You barely have enough room in a decent length query to develop one synopsis. Forget about two. Furthermore, I don’t care about a sequel unless I like the first book. So let’s not get ahead of ourselves.
This isn’t to say you shouldn’t let the agent know that your work is the beginning of a series. But you need only mention it. You can say something along the lines of “KYRA’S AWESOME MANUSCRIPT is a romantic horror and is the first of a planned series of five books.” That’s it. That’s all you need.
I’ll say one final thing in regards to approaching agents who are hesitant about debut authors with series. If you can, pitch your book as “standalone with series potential.” This is a golden phrase, because it says “if this book does well, we can publish more and make even more money. But if it doesn’t we aren’t stuck publishing it.” (I’m sure your book will do well and we’d be happy to publish more, but those pesky people in finance like their contingency plans).
Naturally, you shouldn’t use this phrase if it doesn’t work. You’ll have to ask yourself how cliff-hangery your ending is. Had Harry Potter not become a mega hit, we probably would have been content with leaving the first book where it ends. It can standalone because it ties up the main conflict of the book. Conversely, The Fellowship of the Ring could not standalone because all sorts of bad things are going down when it ends. If that were the only book, we’d all be like, “Geeze, Mr. Tolkien. What gives?”
At any rate, yes, you can ptich a series. But you only need a little mention that it’s a series because you want to focus on the book you’re actually pitching.