You could not protect an Excel file with openxml.
Using password protection doesn't not generate an Office Open XML file and you will get a file in binary file format.
The Open XML SDK does not work with the binary file formats.
One suggestion is that you use Workbook.Password property to set the password:
Here are some similar threads you can refer to.
How to protect document with password using C# and openxml
Protecting Excel file created through OpenXML
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