Cannot export image

I would first try reset the preferences to the default settings.

Reset Procedure:

• Close Lightroom.
• Windows: Hold down [Alt]+[Shift] while restarting Lightroom.
  Mac: Hold down [Opt]+[Shift] while restarting Lightroom.
• Overwrite the Preferences when prompted by the dialog.
• Close Lightroom.
• Restart Lightroom.

It's recommended to backup your preferences before resetting the preferences: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

If this doesn't help we need more infos about your system.

Please give us more informations about your system including the following:

• Adobe product and version number
• Operating system and version number
• The full text of any error message (s)
• What you were doing when the problem occurred
• Screenshots of the problem

Please provide also a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

What has been changed on your system before the error occur?