If your download bar didn’t reach 100% then you’ve had network connectivity problem that disrupt download. The download resume after network failure is not supported so you’ll need to download file from the start.
If your network connection is bad all the time then you can download the template rpm manually from here:
https://yum.qubes-os.org/r4.1/templates-itl/rpm/
Then transfer in in dom0:
And install template manually with dnf:
sudo dnf install /path/to/template.rpm