webpacker has been deprecated (or retired as they call it) and is no longer included in rails 7 by default:

Only security issues will be considered and there will be no version 6 release of Webpacker

For applications currently using Webpacker, the first recommendation is to switch to jsbundling-rails with Webpack.

as a consequence, i think activeadmin should no longer suggest integrating with webpacker. i'll push a PR shortly to remove the relevant section from the installation instructions.

perhaps the use_webpacker config option should start showing a deprecation warning as well?