Buttons in tkinter are not visible

I didn't test it but real problem is command=cl_s() which is wrong.

command= needs function's name without () - command=cl_s - and when you press button then python will use () to run this cl_s.

At this moment it runs cl_s() at once and it runs while-loop which is blocking all code when you set b=True and it can't run .pack() to show button.

When you remove b=True then it skips loop while b: and it can run .pack() to show button.

If you add some print() inside cl_s() and inside while b: then you will see this.
If you add also print() after start = ... then you will see that this is not executed when you set b = True

Mininal working code (with mainloop() to work even without IDLE or Thonny)

But I would also use root.after(1000, function) instead of while-loop and sleep(1) and .update()

import tkinter as tk
import time  # PEP8: preferred to put every module in separate line

# --- classes ---  # PEP8: all classes after imports

# --- functions --- # PEP8: all functions after classes


def cl_e():
    global b  # inform function to use global variable instead of creating local one

    b = False


def cl_s():
    global b  # inform function to use global variable instead of creating local one

    b = True  # need it to run again after pressing `End` and `Start` again

    #print("running cl_s")
    h = 0
    m = 0
    s = 0
    while b:
        #print("running while b")
        if s < 60:
            s += 1
            time.sleep(1)
        elif m < 60:
            s = 0
            m += 1
        else:
            s = 0
            m = 0
            h += 1

        txt = f"{h:02}:{m:02}:{s:02}"  # format numbers with 2 digits, with leading zero

        canvas.delete("all")
        canvas.create_text(250, 250, text=txt, font="Arial 70 bold")
        canvas.update()


# --- main ---

# global b  # variables created outside functions/classes are global
b = True  # this is already global variable

root = tk.Tk()

canvas = tk.Canvas(root, width=500, height=500, bg="white")
canvas.pack()

canvas.create_text(250, 250, text="00:00:00", font="Arial 70 bold")

start = tk.Button(root, text="Start", command=cl_s)
#print("after start")
start.pack()

end = tk.Button(root, text="End", command=cl_e)
#print("after end")
end.pack()

root.mainloop()

PEP 8 -- Style Guide for Python Code

The same with root.after() instead of while-loop.

I have to change if/elif/else because function is repeated always after 1000ms, but original version was running sleep(1) only in some situations.

I also use counter = canvas.text(...) and canvas.itemconfig(counter, ...) to replace text in existing widget without deleting it and creating it again.

And I also use more readable names - running instead of b, clock_start instead of cl_s, and clock_end instead of cl_e

import tkinter as tk
import time  # PEP8: preferred to put every module in separate line

# --- classes ---  # PEP8: all classes after imports

# --- functions --- # PEP8: all functions after classes


def clock_end():
    global running  # inform function to use global variable instead of creating local one

    running = False


def clock_start():
    global running

    running = True

    # clear previous text
    canvas.itemconfig(counter, text="00:00:00")

    # start after 1000ms (1s) with h=0, m=0, s=0
    root.after(1000, counting, 0, 0, 0)


def counting(h, m, s):
    if running:

        s += 1

        if s == 60:
            s = 0
            m += 1

        if m == 60:
            m = 0
            h += 1

        txt = f"{h:02}:{m:02}:{s:02}"  # format numbers with 2 digits, with leading zero

        # replace text without creating widget again
        canvas.itemconfig(counter, text=txt)

        root.after(1000, counting, h, m, s)  # repeat after 1000 ms


# --- main ---

running = True  # this is already global variable

root = tk.Tk()

canvas = tk.Canvas(root, width=500, height=500, bg="white")
canvas.pack()

counter = canvas.create_text(250, 250, text="00:00:00", font="Arial 70 bold")

start = tk.Button(root, text="Start", command=clock_start)
start.pack()

end = tk.Button(root, text="End", command=clock_end)
end.pack()

root.mainloop()