can't use adb shell in recovery mode

I have a china tablet named Quest. I have accidentally repeated pattern lock more than 5 times. Now I have also forgotten the email address.

So for a factory-reset of my tablet I simply booted it into recovery mode. But it shows an Android bot lying with red exclamation mark. After a spending hours I found a shortcut i.e, vol + and power to access recovery menu. But my tablet doesn't go to recovery menu even when I pressed the keys.

So I simply connected my tablet to my pc via USB and installed drivers. In adb command prompt I typed adb devices. Now my tablet is listed as 20110301 recovery. I can simply use only adb pull, adb push, and reboot commands. But I cant access adb shell to unlock my tablet: it simply says exec '/system/bin/sh' failed: No such file or directory (2).

What can I do?