archive command is not working
First, take a pg_basebackup, just in case. Then set your archive command to /bin/true. That will allow Postgres to delete everything to its own satisfaction that has already been deleted. To restore from backup you need the WAL since your last basebackup. Restoring to a point in time before is now impossible I’m afraid. When this is finished, set the archive command correctly. And get some monitoring for this!