You could try making jpeg copies after selecting thumbnails in Organizer:
File >> Export as New
Or use the export button from the LR Library
Choose a folder to save them into.
Raw files are very large for panos and Elements needs to render them a tiff or PSD before [passing them off to the Editor. If source files are too large your system may have insufficient RAM (memory) to complete the merge to panorama. Jpegs are usually more efficient. After exporting as new, open them all in the editor and then start your pano using all open images.