Problem with simple coding for PLDuino: trying to turn relay on and off
For the library warning, you'll need to add / edit library.properties to those folders to specify a valid category.

This is an example library.properties to give you a feel for the structure (I found this particular one here).

name=TMRpcm_PLDuino
version=1.0
author=TMRh20
maintainer=digitalloggers
sentence=PCM/WAV Audio playback (USE_TIMER2 enabled)
paragraph=Play WAV audio files from SD card using AVR (UNO,Nano,etc) based devices.
category=Communication
url=https://github.com/digitalloggers/TMRpcm_PLDuino
architectures=avr


However, these are just warnings, and don't seem to be the cause of your problem.



Your actual problem in this case seems to be that it's trying to upload the sketch via COM4 and it can't find the PLDuino on that port.