Hi John,
I have tested it in my environment. I created an Excel Workbook (.xlsx) in Mac, uploaded it to a SharePoint library, insert an Excel Web Access web part. I can see the web part both in PC and Mac.
Given the error message, this issue may related to the file when uploading it. I’d like to confirm whether such files can be opened in Excel web app. To address the issue, could you please create a test file in Mac and send it to me?
To insert an attachment, please click Use rich formatting when you reply the post. Then click Options to add a File attachment.
Moreover, could you please whether this issuer occur in another site collection?
Thanks,
August