@hosna and @merefield , is SiteSetting.enable_s3_uploads true for both of you? If it is we should just be using SiteSetting.s3_region:
What about SiteSetting::Upload.enable_s3_uploads, what does that give you?
I could do a fix like this, using safe navigation for s3_region:
def self.use_dualstack_endpoint
SiteSetting.Upload.s3_endpoint.blank? && !SiteSetting.Upload.s3_region&.start_with?("cn-")
end
But I want to understand why SiteSetting.Upload.s3_region is blank for you.