There is problem with your networking environment to access https://github.com/nxp/qoriq-engine-pfe-bin.git/.
You could copy the attached tar ball to folder flexbuild_lsdk2108/components/firmware, then execute the following commands.
$ tar -xzvf pfe_bin.tar.gz
$ rm pfe_bin.tar.gz
Continue to run the following command.
# flex-builder -i mkfw -m ls1046ardb -b sd