A few things from your screenshot.
It looks like the active layer is Deckkraft. Not visible and what does it contain ? That big alpha symbol white on black ?
The rectangular selection tool is active, the position is -1111 / 4620 but the size is 0 / 0 - no size ? Is that the actual selection required
Some suggestions
If it is the whole image to select use Select -> All
Make sure that the selection is actually copied Edit -> Copy before pasting.
If you are not sure what you are selecting, turn the visibility of unwanted layers off. Then Layer -> New from Visible Copy that layer.
Use an intermediate step. Make the selection, copy it, paste as a new image Edit -> Paste as a new image If that is what is needed then export that as a jpeg.
It looks like the active layer is Deckkraft. Not visible and what does it contain ? That big alpha symbol white on black ?
The rectangular selection tool is active, the position is -1111 / 4620 but the size is 0 / 0 - no size ? Is that the actual selection required
Some suggestions
If it is the whole image to select use Select -> All
Make sure that the selection is actually copied Edit -> Copy before pasting.
If you are not sure what you are selecting, turn the visibility of unwanted layers off. Then Layer -> New from Visible Copy that layer.
Use an intermediate step. Make the selection, copy it, paste as a new image Edit -> Paste as a new image If that is what is needed then export that as a jpeg.