Incomplete means (and meant) that not all parts are present. The Latin negative in- prefix was already attached, before the word was borrowed. The opposite of uncompleted is completed; i.e, finished, done (of activities). Completed is the past participle of the English verb complete, not a Latin verb.

Is incomplete a real word?

not complete; lacking some part.

Which is correct word incomplete or Uncomplete?

The correct adjective is incomplete. The verb "uncomplete" does not exist. If you want to express that someone took something that had been complete and made it incomplete, you would have to say: He removed the cog wheel from the engine, rendering it incomplete.