why i can't access SU mode !!

[root]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  -Wdate-time 
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat 
-Werror=format-security -Wall
uname output: Linux wael-MS-7592 4.4.0-112-generic #135-Ubuntu SMP Fri Jan 19 
11:48:36 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 48
Release Status: release

Description:
        [Detailed description of the problem, suggestion, or complaint.]

Repeat-By:
        [Describe the sequence of events that causes the problem
        to occur.]

Fix:
        [Description of how to fix the problem.  If you don't know a
        fix for the problem, don't include this section.]