try

.gitlab-ci.yml

@@ -4,7 +4,7 @@ variables:
 pages-master:
   stage: deploy
   script:
-    - mkdir "$EXPORT_DIR/$CI_PROJECT_PATH_SLUG"
+    - mkdir -p "$EXPORT_DIR/$CI_PROJECT_PATH_SLUG"
     - cp -r public/* "$EXPORT_DIR/$CI_PROJECT_PATH_SLUG"
   only: 
     - master