If you face pop-up message for not having permission to do this operation in SharePoint designer or message with following description:
“You do not have permission to do this operation. Ask your site administrator to change your permissions and then try again, or log on with a user account that has this permission. To log on with a different user account click OK.”
I was facing the same issue and after a little on Google, I have found the solution. The reason behind this issue is that you do not have custom permissions for the standard list forms e.g. new, edit, view. These permissions are driven by the base list permissions and New is restricted to only those that can add items to the list.
To fix the issue you must set it as the ‘default’ form for that as shown in below snapshot. SharePoint will only auto-generate links to the default forms. If you only want certain scenarios to use your custom forms, you must direct link for that.
