Unknown variable floor_tex on Gamemaker 8.1. How to fix it?

I have a very trouble with Gamemaker 8.1.

So recently, I've followed this video tutorial which shows creating a game 3D on its game engine.https://www.youtube.com/watch?v=0Jo0JEEukhc

I tried to test my game, but an error message showsthis.What caused that error? Here is the code so I hope you can fix it as well.

direction -= (display_mouse_get_x() -(display_get_width()/2))/5zto -= (display_mouse_get_y() -(display_get_height()/2))/2display_mouse_set(display_get_width()/2,display_get_height()/2)xto = lengthdir_x(100,direction)+xyto = lengthdir_y(100,direction)+yd3d_set_projection(x,y,z,xto,yto,zto,0,0,1)draw_set_color(c_white)d3d_draw_floor(0,0,0,room_width,room_height,0,background_get_texture(floor_tex),room_width/128,room_height/128)d3d_draw_floor(0,0,100,room_width,room_height,100,background_get_texture(root_tex),room_width/128,room_height/128)

Also, the version of Gamemaker is 8.1.141 (r11549)

Thanks
Cheers
-Sajad Sadiq Abd Amhamad