Late June the FreeBSD foundation moved 13.2 to “EOL” status FreeBSD | endoflife.date
As such, it is no longer available at the typical place that the plugin installer is expecting, download.freebsd.org
You should be able to work around the issue by running
iocage fetch --root-dir /pub/FreeBSD-Archive/old-releases/amd64/ --server ftp-archive.freebsd.org
And then selecting 13.2-RELEASE by typing 41.
After you manually pull down the OS you should be able to install as normal in the GUI.