Re: "Pages template "couldn't be downloaded" :
"There is a cloud icon next to the template in the template chooser".
Wondering if that template may have been created (or received) in iCloud, and was not added to the Template Chooser when you created (or received) the template ?
Apple support's online article (link below) may be helpful:
Create and manage Pages templates on Mac - Apple Support
Eg: says: "To Install a template from another source on your Mac":
"You can install a custom template that you received as an attachment by adding it to the template chooser in Pages.
- Double-click the template file (it has the file extension “.template”), then click Add to Template Chooser.
Note: The template is already installed on all your devices and available in the My Templates category of the template chooser if you chose Add to Template Chooser when you created the template (and were using iCloud Drive with the same Apple ID).
You can set Pages to always open new documents from a specific template instead of the template chooser."
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Else: Possibly a Pages update may be waiting to be installed from "Updates" in the App Store ?
or
If needed:
More info. / support / contact (online chat / phone / email etc)
Pages Support website
Pages - Official Apple Support