Learn Dutch: Heeft de douane de dader kunnen vinden die het goud wil smokkelen? - Has customs been able to find the perpetrator who wants to smuggle the gold?

This is the double infinitive rule you are currently learning! When a modal verb like kunnen is used in the perfect tense alongside another verb (here, vinden), it turns back into its infinitive form instead of using a past participle. So, instead of heeft gekund vinden, we say heeft kunnen vinden.

Why are both kunnen and vinden placed together before the second half of the sentence?

In Dutch perfect tenses, the auxiliary verb (heeft) takes the normal verb position, and the rest of the verbs are pushed to the end. When you have a double infinitive like kunnen vinden, they stick together at the end of their main clause, just before the descriptive part (die...) starts.

The word die translates to 'who' here. Can I use wie instead?

No, wie is mostly used as a question word ('Who is that?') or after a preposition. When referring back to a person or thing that was just mentioned (like de dader), you must use a relative pronoun. Because dader is a de-word, we use die.

In the second part of the sentence, why is wil smokkelen at the very end?