You saw a Not a closed polyline message when attempting to bring a closed polyline into your drawing from an Xref using the NCOPY command. (You may have the intention of placing a hatch within the closed polyline.)
The linework you're trying to bring into the current drawing from the Xref is not a true closed polyline (even though it's closed). It's likely a 2D polyline, which will prevent the hatch from placing correctly.
1. Use the NCOPY command on all linework you need to bring into the current drawing.
2. Use the Select Similar option, or select all lines you want to hatch.
3. Explode all selected linework.
4. Use our SUPERJOIN tool, then click one of the lines you brought into the drawing and exploded.
5. The polylines will now be closed boundaries, and you can place hatches within them as needed.