If you did not submit a proof of identity (POI) during the sign-up process and now want to trade US instruments; you need to upload a valid POI via one of the trading platforms by creating a support ticket under the subject Proof of identity / residential address and attaching the document.
If you are a US national you will not be able to trade US instruments. Due to reporting requirements Saxo Bank does not allow US nationals to trade stocks (incl. stock options which settle physically), CFDs, bonds and ETFs issued by US incorporated entities.
In case you are not able to trade US ETFs only, please refer to the following article.