Swagger Editor shows the "Schema error: should NOT have additional properties" error for a path parameter

The error message is misleading. The actual error is that your path parameter is missing required: true. Path parameters are always required, so remember to add required: true to them.

Had the same problem. I accidentally mixed up the syntax from Swagger 2.0 with Openapi 3.0.x. In Openapi 3.0.x, definitions are redefined as components. In the online editor you can click on the button Edit > Convert to OpenAPI 3 to use Openapi 3.0.x.

Read more about components here.

Remark:

OAS 3 is the latest version of the OpenAPI Specification.