Found the issue, it turns out the OpenAPI file for Prefect Server Rest is version 3.1, the regular editor only does version 2.x.x. Was able to open the file in the beta version that can handle OpenAPI 3.1
https://editor-next.swagger.io/ .The file still had two main errors "title and version, required ", once I fixed them by adding the title and version, I was then able to import into Thunder Client(VSCode).
"info": {
"title": "Prefect Server REST API Reference",
"version": "1.0.0"
}