Update CI troubleshooting docs for includes

doc/ci/troubleshooting.md

@@ -246,6 +246,21 @@ If the merge train pipeline was canceled before the merge request was merged, wi
 
 - Add it to the train again.
 
+### Project `group/project` not found or access denied
+
+This message is shown if configuration is added with [`include`](yaml/index.md#include) and one of the following:
+
+- The configuration refers to a project that can't be found.
+- The user that is running the pipeline is unable to access any included projects.
+
+To resolve this, check that:
+
+- The path of the project is in the format `my-group/my-project` and does not include
+  any folders in the repository.
+- The user running the pipeline is a [member of the projects](../user/project/members/index.md#add-users-to-a-project)
+  that contain the included files. Users must also have the [permission](../user/permissions.md#job-permissions)
+  to run CI/CD jobs in the same projects.
+
 ## Pipeline warnings
 
 Pipeline configuration warnings are shown when you: