another U8glib.h error message: 'u8g' was not declared in this scope

hi arduiners!

another U8glib.h error message: 'u8g' was not declared in this scope.

I was trying to install a display LCD12864 to my Anet A8 with the A1284 chip.
how can I fix that problem with the "'u8g' was not declared in this scope" message.

could someone carefully guide me in the right direction please?
I want to install some window frames in the house and need to print some wedges for adjusting the frame-space to the wall.

thanks!

Arduino: 1.8.5 (Windows 10), Board: "Anet V1.0"

sketch\ultralcd_impl_DOGM.h:232:32: note: in expansion of macro 'PAGE_UNDER'

#define PAGE_CONTAINS(ya, yb) (PAGE_UNDER(yb) && u8g.getU8g()->current_page.y1 >= (ya))

^

sketch\ultralcd_impl_DOGM.h:611:8: note: in expansion of macro 'PAGE_CONTAINS'

if (!PAGE_CONTAINS(row_y1, row_y2)) return;

^

In file included from sketch\ultralcd.cpp:96:0:

ultralcd_impl_DOGM.h:644: error: 'u8g' was not declared in this scope

u8g.print(' ');

^

sketch\ultralcd.cpp: In function 'void lcd_update()':

ultralcd.cpp:5344: error: 'u8g' was not declared in this scope

u8g.firstPage(); // Start the first page

^

ultralcd.cpp:5348: error: 'u8g' was not declared in this scope

u8g.setColorIndex(1); // And reset the color

^

Mehrere Bibliotheken wurden für "U8glib.h" gefunden
Benutzt: C:\Users\Anwender\Documents\Arduino\libraries\U8glib
Nicht benutzt: C:\Users\Anwender\Documents\Arduino\libraries\U8glib_Arduino-1.19.1
exit status 1
'u8g' was not declared in this scope

I forgot to mention that I am using Marlin 1.1.9

Could I possibly help with providing additional information?

can someone guide me in the right direction please? I would have to print something urgently.