Apple Mac users who have upgraded to OSX Mojave, have been experiencing problems using the Mendeley Reference Management Tool.
Elsiever have reported that the Mendeley Desktop application is currently incompatible with OSX Mojave, and are currently working with Microsoft to find a solution (https://service.elsevier.com/app/answers/detail/a_id/28441/ & https://service.elsevier.com/app/answers/detail/a_id/18155/kw/mac/p/16075/supporthub/mendeley/related/1/).
In the meantime, Mac users are advised not to upgrade to Mojave, or if they have to make use of the Zotero (free, open source) reference management tool (https://www.zotero.org/).