I created a Collection by mistake, but I can’t find a way of deleting it now. Does anyone know how to delete it?
You cannot currently delete a collection. We’re working on this.
Ok. 
Hey!
You can finally remove unneeded database collections from your site! No more “NewCollectionName”, “Test” or other typos polluting your database folder.
Be aware that removing your collection might have some consequences for your site, so it’s better to remove Dynamic Pages, Datasets, and Reference type fields that are pointing to collection about to be deleted.
More details here: https://support.wix.com/en/article/removing-a-collection-from-your-database .
Happy cleaning!