It should be right there in the pie menu with all of the other options that come up when you click on the workbench. It's been a little while since I've made one, but I believe it should be the option one step counter-clockwise from the top option.
I suppose you could try deleting the bench to see if that helps (I'd grab the scrap stored in it and put it in your sim's inventory). But first, make sure you have some scrap in your bench, and if you don't, put some in there to see if that works. There is something else in the game (what it is escapes me for the moment) that is "free" to do, but you can't do it unless you actually have what you need to do it. It doesn't use the supplies, but unless you have them you don't get the option to do it. I don't know that it will help, but it's worth a try.
