If you were looking for the full name, try searching for part of it (for example "Poa an" instead of "Poa annua").
Try to find the parent taxon (genus for a species , family for a genus, etc.), then check in the "List of subtaxa" section if there is the taxon in the database of the site.
See the list of synonyms for the required taxon on the website GBIF or in other web resources and repeat the search by synonyms.
If you are sure that the taxon absent in the database, read help.