The freedom-u-sdk repo has fully built images that you can download. On the right hand side see the Releases (25) link and click on that, then scroll down to whichever release you want. The 2021.03.1 image is the one that ships with the board, but you may as well try the latest one. Then follow the instructions for writing an image to an SDcard.
The file you want is demo-coreip-xfce4-unmatched-2021.08.00.rootfs.wic.xz672 MB
The freedom-u-sdk repo is very large. It requires a lot of disk space and compute power and memory to build it. If the build failed you should see some errors. Most commonly this is a problem with the clang build which requires about 12GB to complete. If your machine appears to be hung, it might be swapping or thrashing. You can try reducing the parallelism of the build which might help.
PARALLEL_MAKE="-j 4" BB_NUMBER_THREADS=4
You can reduce both of those numbers to 1 if necessary.