Load external font and does not show font in the window

[buran]

If I may suggest something. Instead of using 1 and 2, use bool variable (and meaningful name :))

import pyglet
import time

use_colon = True
 
pyglet.font.add_file('digital-7.ttf')
window = pyglet.window.Window()
window.set_fullscreen(True)
label = pyglet.text.Label('',
                    font_name='Digital-7',
                    font_size=400,
                    x=window.width//2, y=window.height//2,
                    anchor_x='center', anchor_y='center')

def update_clock(dt):
    global use_colon
    label.text = time.strftime("%H:%M %P" if use_colon else "%H.%M %P")
    use_colon = not use_colon

pyglet.clock.schedule_interval(update_clock, 1)
 
@window.event
def on_draw():
    window.clear()
    label.draw()
 
pyglet.app.run()

however using global variables is bad, so a better solution is to define a class and work with it

import pyglet
import time

class DigitalClock:
    def __init__(self, window):
        self.use_colon = True
        self.label = pyglet.text.Label('',
                    font_name='Digital-7',
                    font_size=400,
                    x=window.width//2, y=window.height//2,
                    anchor_x='center', anchor_y='center')

    def update(self):
        self.label.text = time.strftime("%H:%M %P" if self.use_colon else "%H.%M %P")
        self.use_colon = not self.use_colon

def update_clock(dt, digital_clock):
    digital_clock.update()

 
pyglet.font.add_file('digital-7.ttf')
window = pyglet.window.Window()
window.set_fullscreen(True)


digital_clock = DigitalClock(window)
pyglet.clock.schedule_interval(update_clock, 1, digital_clock=digital_clock)
 
@window.event
def on_draw():
    window.clear()
    digital_clock.label.draw()
 
pyglet.app.run()

This is very basic example. the class can be customized at instantiation (font, size, etc.)

As to tkinter&pyglet code not working on Linux, but working on Windows - I suspect it's just one of those OS dependent quirks, not problem with your code per se...
If you insist on using tkinter, why not install the font, instead of trying to load it from file without installation? In this case no pyglet will be needed at all