Try checkout head ref

.github/workflows/prettier.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
         with:
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: ${{ github.head_ref }}
 
       - name: Prettify code
         uses: creyD/prettier_action@v4.6