This parameter is required for Amazon ECS tasks hosted on Fargate. See Runtime platform for more details.

When you register a task definition, you specify the operating system family.
The valid values for Amazon ECS tasks hosted on Fargate are: LINUX, WINDOWS_SERVER_2019_FULL, and WINDOWS_SERVER_2019_CORE.
The valid values for Amazon ECS tasks hosted on EC2 are: LINUX, WINDOWS_SERVER_2022_CORE, WINDOWS_SERVER_2022_FULL, WINDOWS_SERVER_2019_FULL, and WINDOWS_SERVER_2019_CORE, WINDOWS_SERVER_2016_FULL, WINDOWS_SERVER_2004_CORE, and WINDOWS_SERVER_20H2_CORE.