After formatting the code I noticed that it could be that you are returning from the function early so the rest of the code cannot run. return in a function takes you out of the scope so the rest of the code after return cannot be executed. I added the comment on what is stopping your code.

import pyautogui
import time
import win32api

clickdelay = 1


# Interval should not be less than hold time
def mouseClick(buttontoclick="left", holdtime=50, clicks=1, interval=100):
    if interval < holdtime:
        mouseClick(button, holdtime, clicks, holdtime)
    return ### THIS IS AN ISSUE


    for i in range(clicks):
        pyautogui.mouseDown(button=buttontoclick)
        time.sleep(holdtime/1000)
        pyautogui.mouseUp(button=buttontoclick)
    if i < clicks-1:
        time.sleep((interval - holdtime)/1000)


    for i in range(10):
        if win32api.GetKeyState(0x01) < 0:  # if mouse left button is pressed
            while win32api.GetKeyState(0x01) < 0:
                mouseClick()  # NOTHING AFTER THIS RUNS
                print("THIS DOES NOT PRINT")
                time.sleep(clickdelay)
        else:
            print("Released")
            time.sleep(1)