Interruption installation macOS BIGSUR - IMAC 9.1 20' from 2009

The process for installing MacOS BigSur was smooth until the error message .."The installation of macOs was interrupted. The installation requires the download of important elements. These elements cannot be downloaded for the moment.try again later." The machine is on OS El Capitan (the max version according to Mactracker). What do you recommend for finalizing the installation?

submitted by /u/Emergency-Lettuce882
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