archive command repeatly failing for one particular file
If the file is not in pg_xlog, but there is a .ready file in archive_status, then somebody must have removed that file, which is why archival is failing.

Temporarily set archive_command = '/bin/true' until the view pg_stat_archiver indicates that PostgreSQL is happy again, then set the parameter back to its original value.

Perform a new base backup immediately after that.

You won't be able to recover across the point where you have no WAL archives.

Try to find out which of your administrators is stupid enough to delete files from the data directory and send him or her to a PostgreSQL course.