Why can't I connect the nlohmann/json library?

Question

C++ / C#

Hello. For some reason, it is not possible to connect the nlohmann/json library
In the header file and in the *.cpp file it is written: Compilation fails with the following error:
#include <nlohmann/json.hpp>

In file included from /home/doctor/API/apacheconf.h:7:0,
                 from /home/doctor/API/apacheconf.cpp:5:
/usr/local/include/inja.hpp:37:2: error: static assertion failed: nlohmann/json not found.
  static_assert(false, "nlohmann/json not found.");