chore: Add flags error for older Next.js versions to troubleshooting (#…

…419)

src/content/docs/troubleshooting.mdx

@@ -328,6 +328,13 @@ const decision = await aj.protect(req, { email });
 // You can now access the body again later
 ```
 
+### Error: Cannot read properties of undefined (reading 'flags')
+
+If you see this error then you are probably running an older version of Next.js.
+Arcjet supports the current and previous major versions of Next.js [as indicated
+in the SDK reference](/reference/nextjs#requirements). Upgrade your Next.js
+version to the latest version to fix this error.
+
 ## Get help
 
 We provide [technical support](/support) for all Arcjet users so if you need