You can't delete a sheet if it is currently in use on either a dashboard or a tooltip.
Its not an entirely intuitive process but you should go through your dashboards and detach the sheets you wish to delete, once this is complete the delete option will become ungreyed.
The only alternative to this is to kick to the chase and just delete the entire dashboard and start again from scratch, but that might not be in your interests.
Yesterday I had the same issue in Tableau Desktop.
When we get the "internal error" message (maybe from being timed out) and we can't edit/save our work, I guess you've logged out using this little box at the bottom of the Tableau Public editor.
It is blank if you are not logged in, and will have your account name if you are (or were). When we click on it, and select "sign out," our account name disappears. Then when we try to save our work again it takes us through a login process and saves our work. I'd recommend saving your worksheet right away after creating it, before you have done a lot of work on it. That way you will have to log in to your account. When you get timed out, you can log out and then back in again to save your work. It's a silly thing to have to do, but at least it saves my worksheets
Thanks Peter. The problem was not that delete wasn't an option, it was that I got an internal error when I tried.
Thanks Mahfooj. Getting timed out of the server has been a long-standing annoyance fro me, so I regularly save my work. But this is happening when I know I'm logged in.
I did finally mange to delete the sheets by the method of creating a trash dashboard and putting all the sheets I wanted to get rid of on it, and then deleting the dashboard. Not sure why this was not working before, but problem is resolved!