1. Identify the source of the error: PostgreSQL 39P02. This error is caused by a deadlock, which occurs when two or more transactions are waiting for each other to finish before they can proceed.

2. Check the PostgreSQL logs for more information about the deadlock. This can be done by running the following command:

SELECT * FROM pg_stat_activity WHERE state = 'idle in transaction';

3. Analyze the logs to identify the transactions that are causing the deadlock.

4. Kill the transactions that are causing the deadlock. This can be done by running the following command:

SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE state = 'idle in transaction';

5. Monitor the PostgreSQL server for any further errors. This can be done by running the following command:

SELECT * FROM pg_stat_activity;

6. Use an automated database observability tool to monitor and fix PostgreSQL 39P02. Automated database observability tools can help identify and diagnose issues quickly, as well as provide insights into the performance of the database. They can also provide alerts when errors occur, allowing for quick resolution.