My case power button keeps flashing slowly as if the PC is in sleep mode but it’s completely on. Is that normal ?
Welcome to the forums, newcomer!
You've probably wired the LED to be connected to the HDD activity LED headers on the motherboard. Might want to take a picture of the header that it's populating on the motherboard and parse it along for use to see, apart from mentioning the make and model of your motherboard.
Take another picture at another angle so we can see the Dupont connecting block. As for the flickering, might want to include a video of what you're seeing. From my preliminary findings, the front panel looks hooked up the right way.