Hi Jennie,
Thanks again for your quick and helpful reply.
I fixed the properties panel as you suggested but I had to create a new project because on the last one I had to change all the include paths and it was a little messy.
Now I have a new Proj_test with the following tree:

I fixed the properties panel again and now I can include files like "task.h" and "FreeRTOS.h" to my project without changing all the include paths.

The main project file "Proj_test.c" is a copy of the "freertos_blinky.c" example, like I did on the previous project.
In the main project, the "task.h" file is correctly included, but it seems that their functions are not visible from my file.
I saw that the header was called "task.h" and the source "tasks.c", so I tried to rename the source file to match the header but the error still appears.

I tried then to delete those files from the project and to create new ones with the same name and content. No luck again.
I wonder if there is another step to include correctly headers with their relative sources to my project.
Have a nice day, Gioele.