Hi,
I'm Sneha and I'd be happy to help you out with your question. Sorry for the inconvenience caused.
One possibility is that the line is actually a part of the document's formatting, and not a manually-inserted line. For example, it could be a border around a paragraph or a horizontal line separating sections of the document. In this case, you would need to modify the formatting of the document to remove the line.
Also it could be that the line is actually an image or other type of object that has been inserted into the document. In this case, you would need to locate and delete the object in order to remove the line from the document.
If nothing works it can be issue with document best way will be create New file and copy content with Selecting All in one go copy paragraph by paragraph or if you're copying by Select All and Paste, Paste it as plain text.
If you have any other questions or need assistance with anything, please don't hesitate to let me know. I'm here to help to the best of my ability.
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Best Regards,
Sneha