Hey @testing !
You have a good eye!
Actually, 完成する is closer to intransitive verb by itself, meaning ‘to be completed’.
新作映画が完成した。
The new movie has been completed.
In order to make it more transitive, we can use a kind of trick - using causative form.
In other words, using させる will make it transitive so you can use it like this:
絵を完成させる。
I will complete the paining.
Using causative form this way also often adds strong nuance that it will be speaker/writer who will do the verb.
I hope it helps,
Cheers!
PS
You can use this method with other verbs like 終わる、making it into transitive 終わらせる (instead of using 終える). This can be used to emphasize the nuance mentioned above, or to show eagerness to do soemthing.
宿題を終わらせる
To finish homework.