metafor is an R package that's needed to run SSDforR and Mojave is the latest version of the operating system for Apple laptops and desktops. Mojave's versions of developer tools needed to compile programs is not totally consistent with the programs used in packages like metafor. Usually, this isn't a problem because a pre-compiled, "binary," version is also available.
Those binaries are archived only for a few years, at most, and then only the source code is kept. Unless you have more than beginner skills in programming, it is highly unlikely that you will be able to get the source code of metafor to compile on your out-of-date operating system version.
Short answer, if you don't bring your operating system up-to-date, the Mojave version, for which there is a binary metafor you are not going to be able to install SSDforR.