add troubleshooting section for beta.armbian.com apt repo issue

README.md

@@ -172,6 +172,20 @@ Before committing, ensure the following do not contain real credentials:
 
 ---
 
+## Troubleshooting
+
+### `apt update` fails with beta.armbian.com error
+
+On some Armbian images, a beta apt repository is enabled by default and may cause `apt update` to fail. Comment it out:
+
+```bash
+grep -r "beta.armbian" /etc/apt/sources.list /etc/apt/sources.list.d/
+```
+
+Open the file that contains it (usually `/etc/apt/sources.list.d/armbian.sources`) and comment out or remove the line referencing `beta.armbian.com`, then run `apt update` again.
+
+---
+
 ## Requirements
 
 **Spoke:** Armbian (Debian-based), ARM device, Docker, autossh, git