When you convert your document to plain text, all the field codes are gone, and that document can't be converted to EndNote again instantly. When you converted your document to plain text, a copy with plain text was made, and the original document was saved. Now you can go back to your original EndNote document before converting it to plain text and continue with that document.
In the future, when you want to send your document to your supervisor again, use the option of "convert to unformatted citations" under the "convert citations and bibliography" drop-down menu. This will remove the bibliography, and your text references will look like this example: {Abrego, 2019 #206@@author-year}. Just tell your supervisor this is how it will display, but it will display correctly as soon as the document is linked to EndNote again. When you get the document back from your supervisor, click on "update citations and bibliography" Then the field codes will be restored, the text references will display as they should be, and the bibliography will be back in your document.
