"Unbound module Core_kernel" Error when running "dune build"

[FatGabelstapler]

I have multiple unbound value errors when I run "dune build":

nameofuser@iMac flitter % dune build

File "src/colors.ml", line 1, characters 5-16:
1 | open Core_kernel
         ^^^^^^^^^^^
Error: Unbound module Core_kernel

File "src/splits.ml", line 1, characters 5-16:
1 | open Core_kernel
         ^^^^^^^^^^^
Error: Unbound module Core_kernel

File "src/duration.ml", line 1, characters 5-16:
1 | open Core_kernel
         ^^^^^^^^^^^
Error: Unbound module Core_kernel

File "src/big.ml", line 1, characters 5-16:    
1 | open Core_kernel
         ^^^^^^^^^^^
Error: Unbound module Core_kernel

File "src/hotkeys.ml", line 1, characters 5-16:
1 | open Core_kernel
         ^^^^^^^^^^^
Error: Unbound module Core_kernel

File "src/loadsave.ml", line 1, characters 5-16:
1 | open Core_kernel
         ^^^^^^^^^^^
Error: Unbound module Core_kernel

When I run opam list, there is this entry :

core_kernel v0.16.0

I have no idea how I could fix this problem