Aurora PostgreSQL 11 for Amazon Aurora Serverless v1

Hi.

You cannot run PostgreSQL 11 on Serverless v1 as of today.
The only PostgreSQL versions available for Serverless v1 are v10.12 / v10.14 / v10.18.

https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Concepts.AuroraFeaturesRegionsDBEngines.grids.html#Concepts.Aurora_Fea_Regions_DB-eng.Feature.Serverless