After exporting from Darktable, I continue getting a message stating, "IMG...CR2 file is currently unavailable"

I’m about 2 weeks new to darktable. I (almost) successfully imported, edited & exported my current project. When I was done editing in darktable, I noticed that about 32 photos didn’t export into my designated folder on my desktop. When I go back in to export the remaining files, I continue getting a message stating, "IMG…CR2 file is currently unavailable".