We are sorry to hear about the issue.
Can you please try following steps:
• Try with a new catalog that is placed on your desktop and not in Pictures as default location then perform export operation.
• Please check permissions on the folder to ensure we have complete permissions.
• Please try the steps listed as per enclosed article: https://helpx.adobe.com/lightroom/kb/lightroom-and-el-capitan---mac-os-10-11.html , to ascertain if this helps.
• If the above steps still fail, please share the crash log here.
Please let us know if it helps.