Is my translation correct?
但是德军也仅仅在诺曼底部分地区完成了建造，待盟军登陆的时候，工事完成的程度效果远没有达到预期。
But the German army only finished construction in part of the area of Normandy, when the Allies arrived, the extent of the completion of the fortifications was far below expectations.

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Answer

It totally makes sense, I think part of the area of Normandy should be plural. Parts. or partial. arrived can use landed. and  待 implies 直到，等待。until is more appropriate. For reference, But the German army only finished construction in parts of the area of Normandy, until the Allies landed, the extent of the completion of the fortifications/or defensive structure was far below expectations. hope that helps.