这个帖子帮我解决了这个问题:

This thread helps me resolve the issue:

https://developercommunity.visualstudio.com/content/problem/28126/unable-to-add-reference-in-visual-studio-2017-refe.html

总结一下，线程说我需要在缓存中添加某个 .dll 文件，以便我添加引用.不知道为什么会出现这个问题，但我希望它可以帮助任何遇到相同问题的人.

Just to sum it up, the thread says that I need to add a certain .dll file on the cache for me to add References. Not sure why this issue occur but I hope it helps anyone who encounters the same issue.