Before you import a structure from a file or web service, the add in checks to see if it is valid.
This is an example of an error during the import of a chemical structure from ChEBI:
You may also see a similar dialog when you try to import a structure:
The add in reports an error because the add in does not support an atom found in the import file. The add in is able to import from many chemistry formats; not all of the atom and bond types that they contain are supported.