Learn German: Wir wissen nicht, inwieweit man die Lieferkette nachvollziehen kann. - We do not know to what extent one can trace the supply chain.

The word inwieweit ('to what extent') is acting as a subordinating conjunction here. It introduces an indirect question, which forms a subordinate clause. Just like with dass or ob, this means the conjugated verb (kann) must be placed at the very end of the clause.

Could I use inwiefern instead of inwieweit here?

Yes, they are almost entirely interchangeable in this context. inwieweit sometimes implies a slightly more measurable extent, whereas inwiefern leans a bit more toward 'in what way'. However, for most native speakers, both perfectly translate to 'to what extent' in this sentence.

The verb nachvollziehen is separable. Why isn't it separated in this sentence?

It stays together because it is paired with the modal verb kann, which always requires the main verb to be in its full infinitive form. As a quick refresher: even if there were no modal verb here, a separable prefix reattaches to its verb whenever it is pushed to the end of a subordinate clause.

The English translation uses 'one can trace', but wouldn't 'the supply chain can be traced' sound more natural?