Check permissions on the folder you are exporting into. Launch Finder and click on the folder to highlight it. Press Cmd+i to bring up the info dialog. Ensure everyone has read & write privileges. If not, click the padlock icon to change.
I'm losing my damn mind. I've been using LR for years. Just got a new iMac. Set up LR. Imported photos. Edited. No problem. But....... can NOT export. Help!
Tell us what happens. We need details. Lots and lots of details.