Don't try to check if is mergeable, and display read-only warning

app/models/merge_request.rb

@@ -258,7 +258,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def check_if_can_be_merged
-    return unless unchecked?
+    return unless unchecked? && !Gitlab::Geo.secondary?
 
     can_be_merged =
       !broken? && project.repository.can_be_merged?(source_sha, target_branch)

app/views/projects/merge_requests/widget/_open.html.haml

 .mr-state-widget
   = render 'projects/merge_requests/widget/heading'
   .mr-widget-body
-    - if @project.archived?
+    - if Gitlab::Geo.secondary?
+      = render 'projects/merge_requests/widget/open/geo'
+    - elsif @project.archived?
       = render 'projects/merge_requests/widget/open/archived'
     - elsif @merge_request.commits.blank?
       = render 'projects/merge_requests/widget/open/nothing'

app/views/projects/merge_requests/widget/open/_geo.html.haml

+%h4
+  = icon('exclamation-triangle')
+  Merge requests are read-only in a secondary Geo node