SYNTAX ERROR in Pledge Report when Balance Due is selected (ver. 5.18.2)
This is a bug.  It only happens when you have modified the name of the pledge status "Completed" or "Cancelled".  In your case, "Completed" has been modified.

While you could change that label back, the report shouldn't crash regardless.  It was easy to fix, so I reported the issue then made the fix.

You literally need to change one word in one file, so if you feel comfortable changing the contents of a file in your CiviCRM directory, you can look at the fix I made and make the change on your site.  If you do, please report back on Github that it works - that will speed up the process of getting this fixed merged into CiviCRM for everyone.