Just download the macOS mono target support under component installers window in the website
that's what I did when I encounter the same situatuon

@DuckMint You dont have to reinstall the entire editor! You can download the Unity installer and only check mac build support and it will install that component into your existing Unity build.

@DuckMint 13 gbs? It's Entirety Is Only About 7 gbs If You Install Only Unity And Mac Build Support It Will Be About 2.8 gbs

@FairFireFight I wanted to avoid the 13GB reinstall

When First Installing unity , You Get A Bunch Of Check Boxes With Some Things , One Of Them Is Mac Build Support (Mono) . Sorry But You Have To Reinstall Unity From Here
Scroll Down Till You Find Unity 2018.x , Click It , Scroll Down A Bit And Find Unity 2018.3.8 , Click Download (Win) (Or Mac If You Have That) Then Continue

In The Installer , Check Mac Build Support (Mono)

Can I Get Your Mod Tools? Lemme See