add section to common problems section of the apm config page

docs/en/observability/apm/configure/agent-config.asciidoc

@@ -54,6 +54,9 @@ use these {es} options to provide {es} access.
 [float]
 === Common problems
 
+[float]
+==== HTTP 403 errors
+
 You may see either of the following HTTP 403 errors from APM Server when it attempts to fetch APM agent configuration:
 
 APM agent log:
@@ -74,3 +77,10 @@ This occurs because the user or API key set in either `apm-server.agent.config.e
 (if `apm-server.agent.config.elasticsearch` is not set) does not have adequate permissions to read source maps from {es}.
 
 To fix this error, ensure that APM Server has all the required privileges. See <<apm-privileges-agent-central-config-server>> for more details.
+
+[float]
+==== HTTP 401 errors
+
+If you get an HTTP 401 errors from APM Server, make sure you're using a token that you're using
+an API key that is configured to *Beats*. For details on how to create and configure a compatible
+API key, refer to <<apm-beats-api-key-publish>>.