EndNote does not export abstracts when saving BibTeX files. However, you can export other formats (RefMan RIS and EndNote Export) to retrieve the abstracts.
Please choose one of these 2 formats which are fully compatible with Rayyan.
We have documented the EndNote export process here: https://rayyan.ai/guides/endnote which you can also find in the search upload page.
** If you don't have BibTeX, RefMan RIS or EndNote Export as Output Style options, please make sure you have a complete installation of EndNote. You need to select the Export option (as shown in the image below) when you install it.