I was following a Youtube tutorial when suddenly the warp transform tool doesn't work. Specifically, it doesn't warp the first layer compared to other layers. How can I fix this?
There are a couple of possible problems. It's hard to tell precisely what's wrong without access to the XCF file or your setup of GIMP. But anyway, here are some things to check.
The warp tool only works on one layer at a time. If you want to warp all layers simultaneously, you'd need to merge all the layers first.
If a layer has the pixel lock engaged, then you can't use the warp tool on it. Make sure the layer is unlocked.
If a layer above is hiding (covering over) a layer below, then you might not see anything happen when you use the warp tool on the layer below. Unhide the layer above before trying to warp the layer below.
If you have floating selection (check your layers panel), often the result of copying and pasting an image, then you can only warp the floating selection. You won't be able to select another layer for warping until you have either anchored the floating selection, or have promoted it to a layer of its own.
Check the Tool Options for the Warp tool. It's possible you may have accidentally changed a setting preventing the tool from working properly.
It should look something like this shown below. Maybe also try clicking the tiny menu icon at the top right, and choose Tool Options Menu > Reset Tool Options if you can't work out what you might have changed.