You have a file `Spec.tla` which requires some constant C in the form of a tuple, which `.cfg` files cannot accept as a value. If you can't use the TLA+ Toolbox, the idiomatic solution is to write a new "model checkable spec" file which `EXTENDS Spec`, define the constant as an operator `Op` in that file, and then put `CONSTANT C <- Op` in that file's `.cfg`.
How do you name the "model checkable spec" file? I think Apalache does `MC_Spec.tla`, the toolbox calls it just `MC.tla`. Do you do one of those, `MCSpec.tla`, `SpecMC.tla`, or something else?