My Personal.xlsb file was "read only" access. It wouldn't save that it was "hidden" even though the file wouldn't just show up like a regular Excel file.
I had to save it under a different directory (I used Downloads for easy access), closed out Excel, deleted the read only file in the XLStart folder, and moved the new editable personal.xlsb file into the XLStart folder so it would open in the background instead of opening every time I opened an Excel File.
I couldn't do a Save As and just overwrite it, because it had to be open to do that, and it wouldn't let me save over an open document.
In Windows 10, the XLSTART folder, which is used for Excel add-ins and macros, can be found in one of the following paths:
For Windows: C:\Program Files\Microsoft Office\Office\XLSTART
For Mac: /Applications/Microsoft Office/Office/Startup/Excel/
If you’re specifically looking for the personal.xlsb file, you can find it here: C:\Users<User_Name>\AppData\Roaming\Microsoft\Excel (replace <User_Name> with your actual user name).