First of all, ensure that the SQL LRS app is running and that the host and port are configured correctly. If you are running SQL LRS as a Docker image, ensure that the port is exposed.
In addition, if you are using a proper domain name (either via DNS or via a hosts file) or using a proxy server, you may need to adjust configuration for CORS (Cross-Origin Resource Sharing). CORS restricts which endpoints SQL LRS will accept requests from; requests from disallowed endpoints will result in a 403 Forbidden response. Either specify allowed endpoints via LRSQL_ALLOWED_ORIGINS (the recommended method for production) or allow all endpoints via setting LRSQL_ALLOW_ALL_ORIGINS to true.
First of all, ensure that you are indeed executing the SQL LRS image in Postgres mode. The command /lrsql/bin/run_postgres.sh needs to be run as a custom command in order to override the default command, which runs the app in SQLite mode.
In addition, check that you have the appropriate values of POSTGRES_DB, POSTGRES_USER, and POSTGRES_PASSWORD for your Postgres Docker image, and that the respective config values (LRSQL_DB_NAME, LRSQL_DB_USER, and LRSQL_DB_PASSWORD) match up.
See the docker-compose.yml file as a reference for running Postgres SQL LRS via Docker/Docker Compose.
You may want to adjust the LRSQL_POOL_MINIMUM_IDLE config var, as it is set to 10 by default in Postgres mode. (See here for more info on connection pool configuration.)