This morning while working on a project in Photoshop CS4 I was trying to move a folder with the keyboard command up and down. This normally will nudge the selected folder in whatever direction I press. However today I got the message: “Could not nudge the selection because the pixels in a fill layer cannot be modified without first rasterizing the layer.”
So naturally, I googled for a fix and surprised to find no decent answer, I went back to trying something myself. What I realized was that this was the result of using the marque tool earlier and somehow improperly deselecting. If you happen to run into this problem, all you need to do is grab your marque tool (M) and select an area then delect. Or, you can go up to the Select menu, and click “deselect.” This solved the quirky little error for me. Let me know if it works for you too!