The plugin may fail to create metrics in certain cases. One of them may be that the ontology has unsatisfiable classes, which means that there is a class that cannot be instantiated which would cause an inconsistency in OQuaRE. Another reason may be that opening an ontology by URL may cause an error in the redirects, in which case it will display a message "The ontology is not valid. If is is an ontology by URL changing the protocol to 'https' might solve it", if changing the protocol still does not work, try downloading the ontology in a file to load it locally.
The plugin may fail to create metrics in certain cases. One of them may be that the ontology has unsatisfiable classes, which means that there is a class that cannot be instantiated which would cause an inconsistency in OQuaRE. Another reason may be that opening an ontology by URL may cause an error in the redirects, in which case it will display a message "The ontology is not valid. If is is an ontology by URL changing the protocol to 'https' might solve it", if changing the protocol still does not work, try downloading the ontology in a file to load it locally.