Hi Ajay Kumar Thanks for the update. Please follow the below document with reference steps to add the APIs manually.
https://learn.microsoft.com/en-us/azure/api-management/add-api-manually#create-an-api
I see that you are trying to import APIs but it fails with following error "Please specify valid OpenAPI specification file." error message indicates that API definition file is not valid.
The API definition which you're trying to use should pass validation from https://editor.swagger.io/ if the file is not valid, the validation exception will show up when importing in azure portal.
To better understand the exception, you can validate the file in Swagger editor and check against the OpenAPI specification https://swagger.io/specification/.
let me know incase of further queries, I would be happy to assist you.