When filling out tax information, it gives the option to select "I am a US person" or I am not a US person".
I indeed am, however my business is registered in the Netherlands because I live and work here. I have not earned income from a US source in almost 3 years. I have a VAT number for my business here, so would it get messy combining Dutch / US tax laws with agreeing that I am a US person? My fear is that I would be double taxed!
You should talk to your local tax experts and authorities. Clearly there are two ways to go with this decision. You should only choose based on what works best for you, as long as it is a legally permissible choice.
Upwork is a software system, not a legal authority, so it doesn't determine what you are legally allowed to do, and it can't tell you what the best choice for yourself is.