HTML dashboards cannot be edited by Splunk. You'll need to use a different tool to make changes. Depending on the nature of the panel you want to add, you may be able to copy and modify an existing panel.
It doesn't help you now, but in the future consider cloning the dashboard to create a backup before converting it to HTML. Then you'll have something you can edit when it becomes necessary.
--- If this reply helps you, an upvote would be appreciated.