I have been trying to add the Dissonant Whispers spell to a homebrew item (scroll). I've got it in and it shows up with the correct format, but hovering over the spell gives the lock icon and indicates that it is part of the 2014 Players Handbook digital content pack. I have purchased the 2024 PHB, and the spell is in there too, so I assume I should have access to it via that? Not sure how to rectify that. 

I'm assuming you're referring to the tooltip you added in the description. If this is the case you just used the wrong tooltip.

You used:

[spell]Dissonant Whispers[/spell]

Which produces: Dissonant Whispers

You should instead use:

[spells]Dissonant Whispers[/spells]

Which will produce: Dissonant Whispers

EDIT: Ninja'd by that darn spamandtuna again!! One o' these days, pork'n'fish I'mma ninja ya right back! 🤣

Awesome! Thank you!