A clumsy, incompetent person — or an ineffective action — is inept. When you're inept, you don't know what you're doing or just can't get it done.
Someone inept is bumbling, clueless, and ineffective. Inept people are dumb or clueless; they don't understand things. More than that, inept people are bad at what they do. An inept lawyer always loses cases. An inept figure skater wipes out on the ice. An inept postal worker loses mail and puts it in the wrong box. An inept person is downright bad at something. The opposite of inept is competent.