The Sketch, it's from a tutorial? There are a things that could be better.
Currently, the Arduino could output any of 8 possible values "a", "b", "c", "d", "n", "w", "x", "y", but your App Inventor Blocks are only checking for three.
I note that the Sketch is expecting to receive some data from the App - but are you going to send data from the App to the Arduino? If so, the order of the Sketch functions will need to change.
Arduino Send must be slower than App Receive, to prevent the data overwriting itself. Therefore, the delay values in the Sketch should, for now, be increased from 400 to 1000. If necessary, the delay can be tweaked by trial and error later - we just want to get it working at this stage.
After each Serial.print(), follow with an "empty" Serial.println() - This is the Character 10 = Ascii LF = End Of Data that the App code is expecting
Setup Clock 1 Timer in your Blocks:
When the App is Exited, tidy-up: