[Tkinter] Tkinter newly created button does not execute command

[Larz60+]

You need to provide a quit method:

from tkinter import *
 
class Application(Frame):
    def __init__(self, master=None):
        self.master = master
        self.master.geometry('300x100+10+10')
        Frame.__init__(self, master)
        self.pack()
        self.createWidgets()

    def new_button(self):
        print("enable_b")
        self.hi_there.config(state=ACTIVE)
        self.new_button.grid_remove()
         
    def say_hi(self):
        print("hi there, everyone!")
        self.new_button = Button(self)
        self.new_button["text"] = "New BTN"
        self.new_button.grid(row=1,column=0)
        self.hi_there.config(state=DISABLED, command=self.new_button)
 
    def createWidgets(self):
        self.QUIT = Button(self)
        self.QUIT.config(text="QUIT",fg="red",command=self.quit)
        self.QUIT.grid(row=0,column=1)
        self.hi_there = Button(self)
        self.hi_there["text"] = "Hello",
        self.hi_there["command"] = self.say_hi
        self.hi_there.grid(row=0,column=0)

    def quit(self):
        self.master.destroy()

def testit(): 
    root = Tk()
    app = Application(master=root)
    app.mainloop()

if __name__ == '__main__':
    testit()

I also moved the __init__ routine to top (will work elsewhere, but other programmers will always look for it at the top)
added startup code 'if __name__ ...'
and created class variable for delete