This fail shows up if BridgeDb did not have an Ensembl mapping for the given gene or protein. Ensembl is used as source for these mappings.
There is not a single reason why this fail shows up. It can be that the used identifier was valid at some moment in time, but is no longer used. Another reason can be that Ensembl does not (or no longer) support the identifier. Manual curation of the identifier used in the pathway is advised.