但是我收到了这个错误: API key not found, please set option('elsevier_api_key_filename') or option('elsevier_api_key') for general use or set environment variable Elsevier_API, to be accessedby Sys.getenv(&
bookdown::pdf_document2: number_sections: yesbase_format: rticles::elsevier_article---
knitr::opts_chunk$set(echo =真)
This is an R MarkdownI'm trying to prepare an Elsevier articl