@poettli , many thanks for your answer. I have tried some cases to locate the issue. First of all, my selected url for SCIHub is normal, because it can download papers. However, when the paper has two version, namely one is for SCIHub and another one is for arxiv, Zotero always downloads the arxiv version. For example, https://doi.org/10.1109/TWC.2020.3024629.
Also, this might help: https://forums.zotero.org/discussion/94816/the-build-in-pdf-resolver-which-is-configured-as-scihub-does-not-work-for-zotero-6-0