Learn Hardcore Japanese: sono fairu ha daizi dakara, matigaete sakuzyositara dame desu. - That file is important, so you must not delete it by mistake.

A native English speaker may expect an object marker like を, but Japanese often makes something the topic with は and then leaves the repeated object unstated later if it is obvious from context.

Why doesn’t it say そのファイルを before 削除したら?

Because the object is already understood from the topic そのファイルは.

Japanese often omits words that are clear from context. Here, the meaning is naturally:

That file is important, so if you accidentally delete it, that’s bad.

You could say something more explicit, such as:

そのファイルは大事だから、間違えてそのファイルを削除したらだめです。

But that sounds repetitive. Omitting the object is more natural.

Why is it 大事 and not 大事な?