undefined reference to boost::system::generic_category() cmake - c++

Question

I have an undefined reference while trying to compile my Qt5 project.
I am usually pretty confident with using CMake but this time I have a strange error that I can't figure out:
undefined reference to boost::system::generic_category()
/usr/local/include/boost/system/error_code.hpp:223: undefined reference to 'boost::system::generic_category()'
My configurations are:
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Qt5.15
cmake version 3.16.3
After typing whereis boost the outcome wasboost: /usr/include/boost and after applying the great power of dpkg -s libboost-dev | grep 'Version' :) the version is Version: 1.71.0.0ubuntu2
I don't understand what is happening, below an example of how my CMakeLists.txt is structured:
cmake_minimum_required (VERSION 3.1)
project(projectA)
set (OpenCV_DIR /home/to/opencv/build)
find_package( OpenCV REQUIRED )
find_package( Boost COMPONENTS system thread filesystem REQUIRED)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
set(CMAKE_INCLUDE_CURRENT_DIR ON)
INCLUDE_DIRECTORIES(${Boost_INCLUDE_DIR})
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)
find_package(Qt5Widgets)
find_package(Qt5PrintSupport)
#make them into headers
qt5_wrap_ui (UI_HDRS ${UI})
add_executable(projectA main/main.cpp ui/qdarkstyle/style.qrc ${SRCS} ${UI_HDRS} ${UI_SRCS})
target_link_libraries (projectA Qt5::Widgets ${Boost_LIBRARIES} ${OpenCV_LIBS} Qt5::PrintSupport)
add_library(projectA_lib SHARED ${SRCS} ${UI_HDRS})
target_include_directories (projectA_lib PUBLIC "src/" "ui/")
link_directories(${Boost_LIBRARY_DIRS})
target_link_libraries (projectA_lib Qt5::Widgets ${Boost_LIBRARIES} ${OpenCV_LIBS})
I have searched and applied solutions I saw on all possible sources I was able to find such as:
This source but that didn't work.
Also from here it seems that this solution shall be applied:
set(Boost_USE_STATIC_LIBS ON)
set(Boost_USE_MULTITHREADED ON)
set(Boost_USE_STATIC_RUNTIME OFF)
find_package(Boost REQUIRED COMPONENTS system)
# the call to include_directories is now useless:
# the Boost::system imported target used below
# embeds the include directories
project(APP C CXX)
add_executable(APP src.cpp)
target_link_libraries(APP Boost::system)
However that also didn't do any specific benefits to finding the solution.
Other posts I consulted were this, this one but no answer was provided.
This post was useful but that also didn't provide any advice that didn't already know.
Thanks for pointing to the right direction and trying to find a solution.