When you try to create a Viewer of a type that includes entitlement, the Offline Banner Assets field is highlighted in red. It reports the following error when you hover your cursor over it:
"The provided HTML resources .zip is invalid. The uncompressed archive should not take up more than 10 MB. The archive should contain one and only one .html or .htm file, located in the top level of the .zip. Make sure that you compress the contents of the HTML resources, not the folder itself. The archive cannot contain more than five levels of nested folders."
Navigate to the folder that contains your HTML assets and select all of the contents of the folder. Do not select the folder itself. When the files are selected, compress them into a .zip file. This step ensures that they .htm or .html file is at the root of the .zip, not nested inside a folder.