Modify error message for clarity

Removes a double negative

Changelog: fixed

app/models/namespace.rb

@@ -566,7 +566,7 @@ class Namespace < ApplicationRecord
     end
 
     if user_namespace?
-      errors.add(:parent_id, _('cannot not be used for user namespace'))
+      errors.add(:parent_id, _('cannot be used for user namespace'))
     elsif group_namespace?
       errors.add(:parent_id, _('user namespace cannot be the parent of another namespace')) if parent.user_namespace?
     end

locale/gitlab.pot

@@ -39862,6 +39862,9 @@ msgstr ""
 msgid "cannot be modified"
 msgstr ""
 
+msgid "cannot be used for user namespace"
+msgstr ""
+
 msgid "cannot block others"
 msgstr ""
 
@@ -39877,9 +39880,6 @@ msgstr ""
 msgid "cannot merge"
 msgstr ""
 
-msgid "cannot not be used for user namespace"
-msgstr ""
-
 msgid "ciReport|%{degradedNum} degraded"
 msgstr ""
 

spec/models/namespace_spec.rb

@@ -50,10 +50,10 @@ RSpec.describe Namespace do
         ref(:project_sti_name)   | ref(:user_sti_name)      | 'project namespace cannot be the parent of another namespace'
         ref(:project_sti_name)   | ref(:group_sti_name)     | 'project namespace cannot be the parent of another namespace'
         ref(:project_sti_name)   | ref(:project_sti_name)   | 'project namespace cannot be the parent of another namespace'
-        ref(:group_sti_name)     | ref(:user_sti_name)      | 'cannot not be used for user namespace'
+        ref(:group_sti_name)     | ref(:user_sti_name)      | 'cannot be used for user namespace'
         ref(:group_sti_name)     | ref(:group_sti_name)     | nil
         ref(:group_sti_name)     | ref(:project_sti_name)   | nil
-        ref(:user_sti_name)      | ref(:user_sti_name)      | 'cannot not be used for user namespace'
+        ref(:user_sti_name)      | ref(:user_sti_name)      | 'cannot be used for user namespace'
         ref(:user_sti_name)      | ref(:group_sti_name)     | 'user namespace cannot be the parent of another namespace'
         ref(:user_sti_name)      | ref(:project_sti_name)   | nil
       end