Files in /data are not readable by the shell user. You need to use the run-as command, which will work as long as the debug build is installed.
You can check if the file exists:
bash
adb shell run-as com.tayek.tablet.gui.android.cb7 ls files
To copy the file to your PC, you will need to copy the log file to your SD card and then use adb pull to retrieve the file. On Android 5.0+ you could simply do:
bash
adb shell exec-out run-as package.name cat files/tablet.log > tablet.log
This should work on previous (and current) Android versions:
bash
> adb shell
$ run-as package.name
$ cp files/tablet.log $EXTERNAL_STORAGE/tablet.log
$ exit
> adb pull /sdcard/tablet.log
Obviously, replace package.name with your package name (com.tayek.tablet.gui.android.cb7).