Hello from Germany,
My english is not so good but I hope it is enough and you understand me 
First some info, I have a TL-1043nd v3 TL-1043 with Extroot on a USB stick and am followed this guide - extroot configuration.
Which runs also perfectly and without problems...
Now I wanted to make a sysupgrade to the current version and found over the LEDE FAQ this script in the openwrt Forum, to automatically save the config and the packages additionally the automatic update.
When running install.sh download I get the following error message:
Collected errors:
- opkg_download_pkg: Package kernel is not available from any configured src.
- opkg_download_cmd: Failed to download kernel.
- opkg_download_cmd: Failed to download kmod-8021q.
.
.
. - opkg_download_cmd: Failed to download zlib.
Abort, error (255) detected!
Since I apparently no one in the openwrt forum can help or would like, I hope now that you can help me further.
How do I get the script to run or fix the error?
I have assumed that the script works together with the extroot config.
If I am completely wrong and that does not work so, or I have to set up "rootfs on external drive" for it, then let me know.
Then I use the webinterface for the update and load the packages newly.
For "rootfs on external" I can not find any current German guide or howto for it.
Many thanks to all in advance