BloodyStump

Turns out this was a noob mistake. All of the articles I searched for told people to drag it into the icon. I was trying to drag into icons on the left side of the window instead of the right which for some reason my brain failed to see. So if you're an idiot like me, this is why you can't drag items into your bag,

Can't drag items into bags • Version 41.78.16 • Singleplayer • Mods? Still does it without mods • Reproduction steps: Try and drag something into a bag • Still occurs on new save? Yes