inconvertibility has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is convertibility.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the quality of not being exchangeable
“the inconvertibility of their currency made international trade impossible”
Synonyms, antonyms and the broader and narrower relations for inconvertibility come from Princeton University’s WordNet. Full sources and licences.