Issue with "undo" in PSP 2020 Ultimate

[caspia1]

Recently I find that the "undo" icon is blanked after I've inserted a layer into another image and wish to undo a mistaken control (eg saturation, hue, etc)
I´ve gone into preferences/undo and the box is clicked ok.
How do I remedy this, please?

[LeviFiction]

I'm not sure I understand what you mean. Can you give precise steps for recreating this?

Here's how I'm interpreting it.
1) You have an image with a layer, and you have a second image
2) You added an adjustment of some kind, we'll say Saturation for now.
3) You dragged the layer from image 1 to image 2.
4) You are now on Image 2 and you want to undo the saturation adjustment you made in Image 1 on the layer in image 2.

If this is correct, then you won't be able to undo that saturation edit in Image 2 as it didn't happen in image 2. Dragging a layer is the same as copying the layer and pasting it as a new layer. So it's a brand new layer. There is nothing to undo except for adding the layer in the first place.

Or do you mean that Undo becomes unavailable on Image 1 the source of the layer?

[caspia1]

Somehow, the issue has resolved itself. Thank you for your advice.