Hi JoelStutz,
First, my pleasure to assist you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
Normally, this error message will prompt out if you do not have your file saved with the correct permission settings.
I search a lot and find some users also mentioned, yes if the file name includes "/"mark (as like you mentioned) it will return the error as you encountered. Please use other marks instead for the file name.
This article is in regard to SharePoint, but I recommend this advice in general for all applications and file systems. The article doesn't say so, but in addition to file names, avoid these characters in sheet tab names in Excel, and in column headers in any data table.
Meanwhile,, please confirm with us the following information:
1.Please help us check if this issue is occurring on other Office for Mac apps. Such as Word, PowerPoint? Does the same scenario occur when you save to the same location?
2.Could you try changing name of the file and save it to that location?
3.A screenshot of your current Office 365 version and Mac version.
I appreciate your understanding and stay safe!!
Best regards
Waqas Muhammad