Quick take: 9 times out of 10, you’re not on a simple pixel layer. Make one layer named “Layer 0”—then try again.

First, the ActionSeps™ baseline (do this once):

  • One layer only. Flatten or stamp, then delete extra layers.

  • Unlock it → “Layer 0.” If it says Background, double-click it (or click the lock) to convert to Layer 0.

  • Rasterized pixels. No Smart Objects, no live effects. Right-click → Rasterize Layer.

  • RGB / 8-bit. Image ▸ Mode ▸ RGB Color + 8 Bits/Channel.

If you’ve done that and it still barks:

  • You’re on a mask/channel. Click the layer thumbnail (not the mask). Press ESC to exit Channels.

  • Smart Object hiding? Layer ▸ Rasterize ▸ Smart Object.

  • Sanity test: Select ▸ Color Range (or Select ▸ All). If that works, rerun the action.

See also: /levels-not-working-photoshop, /need-help-separating-actionseps