UpdraftPlus uses what Dropbox calls an “app folder”. This means that UpdraftPlus only has access to its own files – not any other part of your Dropbox.
App folders are all stored in the top-level folder called “apps”, with their own name. In the case of UpdraftPlus, your files are in “apps/UpdraftPlus”. Dropbox does not allow us to access anything outside of there.
If you purchase the “Dropbox folders” add-on, then you can store your backups inside sub-folders of that directory, e.g. one site in “apps/UpdraftPlus/mysite”, another in “apps/UpdraftPlus/anotherone”, etc.
If you cannot yet see your backup in your Dropbox folder, then you should go to the Dropbox website, and look directly in your Dropbox in there. If you have been viewing your Dropbox on your PC/Mac, then it may not yet be synchronised (it takes time for new data in your Dropbox to get synchronised to the folder on your PC/Mac – the Dropbox website has the “official” contents). Please do check this before you conclude that your backup hasn’t worked!