Preview Unavailable for this file - import failed

Computer : iMac early 2008 running OS X El Capitan V10.11.6

Camera: Nikon D3400 - importing via card reader from SanDisk miniSD

If I import the photos to the lightroom web app, they all import fine. But when I try to import any RAW files to the desktop app, I get the error message "Preview Unavailable for This File" and if I try to import one of the photos I get an error message saying that Lightroom does not support the file version.

I have tried some of the other fixes I have seen on here such as Preferences>Performance>uncheck use Graphics Processor, recalibrating my screen, restarting, uninstalling and reinstalling... I'm not sure what else to do.

Please help!