It appears you are using firmware that is not from the official OpenWrt project.
When using forks/offshoots/vendor-specific builds that are "based on OpenWrt", there may be many differences compared to the official versions (hosted by OpenWrt.org). Some of these customizations may fundamentally change the way that OpenWrt works. You might need help from people with specific/specialized knowledge about the firmware you are using, so it is possible that advice you get here may not be useful.
Ask for help from the maintainer(s) or user community of the specific firmware that you are using.
Provide the source code for the firmware so that users on this forum can understand how your firmware works (OpenWrt forum users are volunteers, so somebody might look at the code if they have time and are interested in your issue).
If you believe that this specific issue is common to generic/official OpenWrt and/or the maintainers of your build have indicated as such, please feel free to clarify.
The AUC package looks for firmware upgrades at openwrt.org. Your model doesn't have support by the official OpenWrt project, so no firmware can be found, thus the error you are seeing.
The only available firmware is the highly modified fork of OpenWrt that GL-Inet builds for your device.
Slate AX aka AXT1800 is not supported by official OpenWRT.
Your firmware is latest from them (171 patches after OpenWRT stopped supporting rev 21.02) https://dl.gl-inet.com/release/router/release/axt1800/4.6.2
You need to ask them to backport any security fixes in packages, as 21.02.x package collection is coming of age. Remove AUC, and never ever replace their core repo.