Getting error on a particular query on a table in Postgres
CLOG is the PostgreSQL Commit Log Manager (CLog). It's telling you here that the record of transactions is fuxed. This may have been caused from any kind of corruption. I would back up the database before attempting anything. There is one suggestion the lists,


  There is a commit log corruption in your database. You need to make the
  files with right size (256K of zeroes). A suitable dd from /dev/zero will
  accomplish this on modern Unixen (ie, anything that has /dev/zero). Filling
  of those files with zeroes nothing but  "all those old transactions are
  aborted".
  
  The safest way to resolve the issue by using the below command:-

dd if=/dev/zero of=Transaction_missing_file_of pg_clog bs=256K count=1

  
  (To make the uncommitted record as they haven't been committed).


This is followed up with the amusing reply,


  Just for the record, playing with pg_clog files like this is an
  insanely unsafe thing to do. Notably it will make all aborted
  transactions in that range become committed, leading to all sorts of
  fun. This is a desperate measures only action and not one that looks
  immediately related to the problem.


I would try backing up, upgrading to PostgreSQL 11, running VACUUM FULL, and restarting before I messed around with this in 9.6. But I would give it a shot if that failed.