A workaround is to enter this into the Terminal prior to running the installer:
sudo nvram IASUCatalogURL=https://swscan.apple.com/content/catalogs/others/index-10.16seed-10.16-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog
kmcbride
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@ts85 answer fixes the issue, but they made a typo. Enter this into the Terminal prior to running the installer:
sudo nvram IASUCatalogURL=https://swscan.apple.com/content/catalogs/others/index-10.16seed-10.16-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog