In order to add a missing abstract, first, go to the research tab on your profile and scroll down to the respective research item that you want to edit. Then, click on the downward-facing arrow below the research item's title and select "Edit". A window of the research item details (including abstract) will appear, and you can change them accordingly. Finally, click "Save" to update the research item with the executed changes. Sincerely,
This is very strange. I never had such a case that I could not add an abstract. Are you able to edit the other data of this publication? If not, are you sure that this publication is in your account, i.e. that it was added automatically or that you claimed authorship? What is the type of this publication - article, chapter, book, data, ...?