I find this sentence quite confusing. 完成 means the act of completion, so I don't understand why だ is used instead of a variation of する. The meaning seems different than a sentence like 今は勉強だ where it says that something is in the process of the する verb rather than being in the state after the する verb as implied in the above example.
Also, when a character says 了解 after receiving a command, are they omitting した or です? It's very complex since there are times when a character will say "これを召喚！" and it means "I will summon this" or "I summon this" and despite following the format of omitting something, they mean different things.