I have a question regarding submitting my work to literary agents. I write juvenile horror novellas for ages 8-14 (I like to think between Goosebumps & Twilight Zone) and what I'm finding is that several agents don't represent novella writers. Is this pretty standard or am I unfortunately finding only those that don't?
You're confused about what you're writing. You're not writing novellas. You're writing chapter books. Novellas are shorter than novels, but that only applies to adult trade books.
You're writing for kids. That means you look for agents who say they are looking for MG (middle grade) or YA (young adult)
You describe your work as scary chapter books akin to R.L. Stine.
And I'm guessing you don't belong to SCBWI because you didn't know this. Join. Learn. It's a resource you'll come to value a great deal.