Hmm, I guess this is coming from sfdisk removing the trailing setup partition. But the next steps, the expansion of the root partition and filesystem are missing.
Can you try to rerun the script and see whether this is now done?
sudo /var/lib/dietpi/services/fs_partition_resize.sh
However, actually you are trying to install an incompatible kernel. This is not going to work, unless you edit boot scripts and environment as well. Didn’t you instead follow the steps I posted in the other topic, to upgrade to a newer compatible Armbian-based kernel build?