Comment 4 for bug 68397

Thanks for your report. At present, you must reformat system partitions (e.g. /, /boot, /usr, or /var) during installation; the fact that the partitioner doesn't warn you about this is also bug 47046, and is fixed in 6.06.1. If you are upgrading an existing installation, then please see http://wiki.ubuntu.com/DapperUpgrades rather than using this installer.

You seem to have plenty of space, so perhaps you can work around this by not subdividing your installation into quite so many partitions. At least to start with, the 3GB you've allocated to / is perfectly adequate to hold /usr and /var as well, so you can always move things around later.