Short answer: Use "Edit->Paste as->New Layer"
Long answer:When you have a layermask attached to a layer, either the mask or the layer itself may active (but not both). If the layer is active, there will be a white border around its thumbnail preview int the Layers Dialog; and the border around the layermask will be black.
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File comment: Editing layer, not the layermask
SSlayer-active.png [ 16.76 KiB | Viewed 5739 times ]
If the layermask is active, the border of its thumbnail will be white and the border of the layer's thumbnail will be black.
Attachment:
File comment: Editing mask, not the layer
SSmask-active.png [ 16.78 KiB | Viewed 5739 times ]
You can make either the layer or the layermask active by clicking on its associated thumbnail preview (or by toggling the state of the "Layer->Mask->Edit Layer Mask" menu item).
In order to get the error you describe, it means that the layermask was active when you performed your Copy or Cut. If that is what you wanted, then use the aforementioned Paste As->New Layer command. But be aware that if you'd wanted to be copying from the layer itself, you need to go back and activate the layer then redo your Cut or Copy.