An open vector path has endpoints that are not connected. That is fine for a line engraving. To cut a shape out of the sheet or engrave a filled area, its contour must be fully closed.
When does a gap cause a problem?
A break in a cutting contour can leave the part attached to the sheet. For area engraving, the software needs to identify the region inside the shape. A small gap may be difficult to see on screen, but it still exists in the file.
Inspect the endpoints
Select the shape and switch to node or outline view. Zoom in on corners, imported curves and places where segments meet. A fill can make an open shape appear closed, so inspect the paths themselves.
Connect the correct points
Use the command to join endpoints or close a path. If the points are apart, it may create an additional line. Check whether that line follows the contour you want. On a curved shape, a straight connecting segment may introduce a new error.
If the shape consists of separate segments, connect them into a continuous contour. Grouping objects is not the same as joining endpoints: a group can still contain open paths.
Check the engraved area after upload
Assign area engraving and inspect the fill, including holes and internal shapes. Correct the source if the fill is missing or unexpected. Read about preparing area engravings and see ordering help for troubleshooting.