I think you mean that the word "usable" is not included in the RAE dictionary. It does, however, exist and is used in Spanish, as the Language Academy itself says above (see link in my previous post). Truly enough, the word is much less common than its equivalent "utilizable".
Thanks for your interest in our dictionaries. As already mentioned, the adjective usable does exist in Spanish. However, according to Ngram Viewer, utilizable is more common, so I've changed the order in which these translations are displayed and now utilizable will be the main option. Changes will be visible once our databases are updated in the next months.