Hi, welcome to the forum.
In your screenshot of the Export settings, you have specified "Original" as the Image Format in the File Settings panel. Because "Original" is simply a copy of the original file plus metadata, anything that changes the pixels (such as Image Sizing, Output Sharpening and Watermarking) is not allowed. Hence those options become non-selectable. Change the Image Format to something other than Original and you'll be able to add the watermark again.