It's under construction at the moment so it's out of use.
「今、ここは作業(工事)中です」= It's undergoing building work. / The toilet is out of order. / It's under construction at the moment so it's out of use.
ボキャブラリー
undergoing = ~中
building work/construction = 工事
out of order (トイレ、機会などが)使えない状態
at the moment = 今
out of use = 使用できない