There appears to be two problems.
Just double checking – you ran that on your PC? If so, your PC is not using Pi-hole for DNS, it's using Comcast's DNS. I assume that Comcast is your ISP which implies that the DHCP server in your router is handing out that DNS server.
You need to modify your router's DHCP settings so that it is handing out only the Pi-hole's IP address for DNS. You may want to disable IPv6 too, at least temporarily while you're diagnosing this.
Another option here is to disable the DHCP server in your router and enable the DHCP in Pi-hole in Settings > DHCP and let that handle it, since it defaults to itself as DNS server for clients.
The other problem seems to be that, out of the box, your Armbian installation is having its nameservers set to Comcast's IPv6 DNS servers but seemingly no IPv4 addresses. It appears that you don't have an IPv6 path to those external servers and so name resolution fails, and this is the cause of your Pi-hole terminal error and your adlist failing to be retrieved. You've worked around this by manually adding in Cloudflare's IPv4 server, and since you do have IPv4 connectivity out, that works and gives you name resolution.
It's likely that somewhere in the Armbian installation you can configure or override this setting and ensure that it has a working IPv4 server as standard.
It's not clear how this is all working together but some things you can try are, again disabling IPv6 in your router temporarily to ensure that IPv4 is the only scheme available and thus getting that working initially. If you are using a Raspberry Pi then perhaps try a standard Pi OS Lite image to confirm a working setup, since the workflow for that distro is standard on that board.