Time Machine not working

Time Machine on my iMac (Mojave) has not backed up for several days. Restarting doesn't help. The external HD 'Time Machine Ext' is connected; its icon appears on the desktop.

Message says:

"Time Machine couldn't complete the backup to "TIME MACHINE Ext"

Unable to complete backup. An error occurred while creating the backup folder."

I would attach the screen shots but there does not seem to be a way to do so.

Thanks for any help!