Add Note about Praefect database tables not being created on initial reconfigure

doc/administration/gitaly/praefect.md

@@ -161,6 +161,34 @@ node, using `psql` which is installed by Omnibus GitLab.
 
 The database used by Praefect is now configured.
 
+If you see Praefect database errors after configuring PostgreSQL, see
+[troubleshooting steps below](#relation-does-not-exist-errors).
+
+#### Relation does not exist errors
+
+By default Praefect database tables are created automatically by `gitlab-ctl reconfigure` task.
+However, if the `gitlab-ctl reconfigure` command isn't executed or there are errors during the
+execution, the Praefect database tables are not created on initial reconfigure and can throw
+errors that relations do not exist.
+
+For example:
+
+- `ERROR:  relation "node_status" does not exist at character 13`
+- `ERROR:  relation "replication_queue_lock" does not exist at character 40`
+- This error:
+
+  ```json
+  {"level":"error","msg":"Error updating node: pq: relation \"node_status\" does not exist","pid":210882,"praefectName":"gitlab1x4m:0.0.0.0:2305","time":"2021-04-01T19:26:19.473Z","virtual_storage":"praefect-cluster-1"}
+  ```
+
+To solve this, the database schema migration can be done using `sql-migrate` subcommand of
+the `praefect` command:
+
+```shell
+$ sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml sql-migrate
+praefect sql-migrate: OK (applied 21 migrations)
+```
+
 #### PgBouncer
 
 To reduce PostgreSQL resource consumption, we recommend setting up and configuring