I believe the iperf application is an extra that is not included in the Xilinx provided sstate-cache. We (Avnet) add this application to the BSP because it is a handy utility to measure Ethernet performance either either a LAN or across the internet. It is an optional application - not necessary or critical for the Linux system. To get you past this error and to hopefully get the BSP to build successfully you can simply disable this app in the rootfs config:
$ petalinux-config -c rootfs
Then search through the menus to find iperf and disable it.
Alternatively, you can also manually edit the <petalinux project>/project-spec/configs/rootfs_config file:
Search for and change "CONFIG_iperf=y" to "# CONFIG_iperf is not set"
I hope this helps. Good luck!