Learn Dutch: Dat kledingstuk mist een knoop. - That garment is missing a button.

You move the subject to the very front and immediately follow it with a pointing pronoun. Because kledingstuk is a neuter (het) word, you use the pronoun dat: Dat kledingstuk, dat mist een knoop. (That garment, it's missing a button.)

What if I wanted to make the button the main topic using Left Dislocation?

Let's assume you are talking about a specific button (die knoop). You pull it to the front and use the pronoun die: Die knoop, die mist dat kledingstuk. Notice that because the object is now at the front, the rest of the clause uses inversion: verb (mist) followed by the subject (dat kledingstuk).

Doesn't missen usually mean "to miss" emotionally, like missing a person?

It does, but it is also used exactly like the English "to lack" or "to be missing [something]". While you could use a verb like ontbreken (to be missing/lacking) here, using missen as a transitive verb is very common and natural for this situation.

Why do we use dat instead of die for kledingstuk?