Fortran module and subroutine

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FORTAN

hi all!
i am new to fortran and I'm using a module that contains a subroutine, that I need to call in another main file, however I am getting some errors and the subroutine is not printing what I want her to print.
Here is my module:

module prestel
use Constants                                                                   
implicit none                                                                              
contains
  subroutine gradient_density(rho,N,dr,gradiant_rho)
    implicit none
    real(kind=dp), allocatable  :: rho(:) !array for volume density
    real(kind=dp),allocatable   :: gradiant_rho(:)
    integer,intent(in)          :: N
    real(kind=dp),intent(in)    :: dr
    integer                     :: i

    do i = 1,N
      gradiant_rho(i) = (rho(i+1) - rho(i))/dr
    end do
    print *,gradiant_rho
  end subroutine gradient_density
end module prestel

and this is my main file:

program pt1
  use prestel
  implicit NONE
  ! Declare input/output variables
  integer                    :: Npts=100
  real(kind=dp), allocatable :: gradro(:)
  real(kind=dp), allocatable :: v_density(:)
  real(kind=dp)              :: rho_0 = 10**17
  integer :: i,j
  real (kind=dp) :: c = 2
allocate(gradro(N),v_density(N))
do i = 1,Npts
    v_density(i) = rho_0
  end do
call gradient_density(v_density,Npts,c,gradro)
deallocate(gradro,v_density)
end program tp1

When compiling my module I get this:

Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

Then, I'm getting this error while compiling my main file:

Undefined symbols for architecture x86_64:
  "___prestel_MOD_gradient_density", referenced from:
      _MAIN__ in cc5vaf5l.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

What I have tried:

I doubled checked my subroutine but i really can't seem to find what's the problem.
Any idea how to solve this?
Thanks in advance!