Some internet service providers, particularly in UK, fail to provide service where Sci-hub is involved. You may need a VPN. Or, contact someone at a university (library staff are usually eager to help) and get them to retrieve the article by the usual means. Or, a regular internet search on the DOI gets you to the regular publisher, who will be little help, but e-mailing an author identified there will likely get you a PDF.

Academic journal publishers are the scum of the earth.