@slashedbeauty Hey there! Currently, there is not a button available in the Classic Editor that inserts the [web_stories_embed] shortcode.
You’ll have to manually enter the [web_stories_embed] code for a single story for the time being. The shortcode can be found again by going to your story, Switch to draft > Publish > Add to new post, then copy that over into the post you are updating.
I have created this feature request to check if there is a better way to fill out and insert this single-story shortcode into existing posts when using the Classic Editor. You are welcome to follow that ticket for any progress/ updates.