+) There is no problem with exporting in the light room mobile cc application, but because the dpi is small, I want to export from the PC version
DPI (or more correctly PPI; pixels per inch) has nothing to do with image size (in pixels). It is only some information for the printer, how large to print each pixel. And it can simply be ignored by a printer, which is what usually happens anyway. Compare it with a chess board. Each chess board has 64 squares. You can make a large chess board or a small chess board, but the number of squares is always the same. PPI only says something about the size of the individual squares. For a physical chess board this may still be important information, but for a digital chess board this is irrelevant because you can decide to change it or ignore it any time you want.
Sorry that you're getting an error message "Some images could not be exported at the size you requested because we could't access the full size original." while trying to export images from Lightroom desktop. Let's help make it right.