adb locat - cannot redirect to textfile - read only filesystem
adb logcat on your phone launches an ADB server on your phone (which is also running on your PC) and connects to itself.  

Then you try to write everything to /logcat.txt (/ is the the root of your filesystem; you can't write there).

Try routing your output to something like /sdcard/logcat.txt or run it directly on your PC, where it gets saved in the current working directory of your cmd.exe.