Install libobjc-6-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libobjc-6-dev
Description:
Runtime library for GNU Objective-C applications (development files)
This package contains the headers and static library files needed to build GNU ObjC applications.
Homepage: http://gcc.gnu.org/
Version: 6.4.0-17ubuntu1
Section: universe/libdevel