You get this error if you don’t include a source file by it’s module name to the list of modules in your .cabal file. Most of the time you get a GHC error saying you should include the module in the cabal file and it works anyway, but not always (could be that it doesn’t work with exposed-modules).
HLS works without that, the samé just happened to me some days ago.
Looking at your project, your Calculator. LinearExpressions module is not mentioned in the cabal file - you also see that in the (mangled) names of the linker error