so I cant create a file system on it.
That's since you already created the filesystem already
Just click on the partition, then 'Mount' and you're done. I know it's confusing since usually you deal with disks different than the one the rootfs is on.
BTW: I hope you know what you're doing since 'SD card reliability' could become quickly an issue if you start to write lots of data to it. Please check the links at the top of this post carefully: https://forum.armbian.com/inde…/954-sd-card-performance/