Not up to and Unable

Unable and not up to are semantically related in untrained topic. In some cases you can use "Unable" instead an adjective phrase "Not up to", when it comes to topics like helpless, not ready. popular alternative

Unable adjective - Lacking qualities (as knowledge, skill, or ability) required to do a job.

Not up to and unable are semantically related in skill topic. You can use "Not up to" instead an adjective "Unable", if it concerns topics such as not able. popular alternative