Tablet Acer Iconia A1-B71

Dear Users,
I have a tablet - Acer Iconia A1-B71, which works fine but I dont have root permission what is very annoying when I want to do something with Files Manager or Total Commander. I readed this thread:

But for my bad I get errors. Below is a log file from CMD:

The most important part is this (I think):
Enter a selection:
system.img.gz found in folder 'system_image'.
/data/local/tmp/busybox telnet 1234
chmod 777 /cache
chmod 777 /cache/system.img.gz
shell@android:/ $ /data/local/tmp/busybox telnet 1234
telnet: can't connect to remote host ( Connection refused
1|shell@android:/ $ chmod 777 /cache
Unable to chmod /cache: Operation not permitted
10|shell@android:/ $ chmod 777 /cache/system.img.gz
Unable to chmod /cache/system.img.gz: Permission denied
10|shell@android:/ $ exit
failed to copy 'system_image\system.img.gz' to '/cache/system.img.gz': Permissio
n denied
opendir failed, Permission denied

Failed to copy system.img.gz. Make sure the folder 'system_image' of this toolki
t contains a system.img.gz suiting to your installed firmware.
Press Enter to leave Application...

If you can tell me - Am I doing something wrong? Is there any solution?

Thank you for the smallest help.

Kind Regards,