Undefined reference to function that is already defined in my header file

I have three files, posl.h, state.c and main.c. What I plan on doing is adding functions that are used throughout the project in posl.h, and then I call it to test the function with main.c, and then make the function in state.c. The problem I am having is that I keep getting an error of undefined reference to init_poslState() even though I have it defined in posl.h

main.c

#include <posl.h>

int main(int argc, char * argv[]) {
    pState poslState = init_poslState();
    return 0;
}

posl.h

#ifndef POSL_LANGUAGE_H
#define POSL_LANGUAGE_H

#define POSL_MAJOR_VERSION 1
#define POSL_MINOR_VERSION 0
#define POSL_RELEASE_VERSION 0

// State
typedef struct POSL_STATE {
    // ...
} pState;
pState init_poslState();
void free_poslState(pState poslState);

#endif

state.c

#include "state.h"
#include <posl.h>

pState init_poslState() {
    pState newState;
    return newState;
}

Makefile

CFLAGS=-g -Wall -Wextra -I./include
CC=gcc $(CFLAGS)
CORE_O_FILES=./src/Core/lexer.o ./src/Core/parser.o ./src/Core/state.o
CLI_O_FILES=
O_FILES=$(CORE_O_FILES)

# Making CLI Tool
posl: $(CLI_O_FILES) libposl.a ./src/CLI/main.c
    $(CC) -o posl -L./ -lposl ./src/CLI/main.c $(CLI_O_FILES)

# Making Library
libposl.a: $(O_FILES) ./include/posl.h
    ar rcs libposl.a $^


# Core Files
./src/Core/lexer.o: ./src/Core/lexer.c ./src/Core/lexer.h
    $(CC) -o $@ -c ./src/Core/lexer.c
./src/Core/parser.o: ./src/Core/parser.c ./src/Core/parser.h
    $(CC) -o $@ -c ./src/Core/parser.c
./src/Core/state.o: ./src/Core/state.c ./src/Core/state.h
    $(CC) -o $@ -c ./src/Core/state.c

# PHONY List

.PHONY: all
all:
    make update-libs
    make libposl.a
    make posl
    make pcc

# Post-Compile Clean
.PHONY: pcc
pcc:
    rm -rf ./src/Core/*.o
    rm -rf ./src/CLI/*.o

.PHONY: clean
clean:
    make pcc
    rm -rf ./libposl.a ./posl*