Chris Thanks for reaching out. From the description I understand that you are trying to import an OpenAPI 3.1 document in JSON format, but you are receiving an error message saying that "prefixItems" is not a valid property. This error message indicates that there might be an issue with your OpenAPI document.
there are some limitations and restrictions when importing an API. You can find more information in the following document: https://learn.microsoft.com/en-us/azure/api-management/api-management-api-import-restrictions
To validate your OpenAPI document locally before importing it to APIM, you can use a third-party tool such as Swagger Editor: https://editor.swagger.io/
please verify and let me know incase of further queries, I would be happy to assist you.