This will happen sooner or later in one of your weed identification sessions. If there are no weed species listed in the Entities Remaining panel, this simply means that no taxa in the database match the selection of feature-states you have made. There are several explanations possible.
The most common is that the plant is not a weed or that you have made an error in one or more Feature-states that you have selected. Therefore, review the feature-states you have chosen and decide which ones you are uncertain about. Try unselecting the uncertain states one by one to see what effect this has on the species remaining; whether one or more weed species move back into the Entities Remaining panel. Review the fact sheets and images for these remaining species to check whether you made an incorrect decision about a feature-state. and check on any information in the fact sheets about native species that might be confused with these remaining weed species.
If this does not result in an identification, it may be that the weed you have found is not currently recorded in the State or Territory you selected. Unselect that State/Territory and see what effect that has on the Entities remaining. There is the unlikely possibility that you have found a species not previously recorded in your State/Territory or, if there is still no remaining weed species in the Entities Remaining panel, it may be a new record for Australia – in either case you should report this to the appropriate authority.