Joshwa You need to provide the TIN (tax identification number) of your country of residency and for every country in which you‘re obliged to pay taxes.
In The Netherlands, this TIN is called BSN. In other countries, it will have a different name.
SSN sounds like you‘re maybe talking about the social security number from the USA? As a US person, you’re always liable to pay or at least declare taxes to the US government, even if you do not live there.
In that case, bunq will need both your SSN from the US as well as the TIN from your current country of residency. As soon as you register in a European country, you will get a TIN from that respective government.
If you’re having issues locating it, tell us the country and someone might be able to give more detailed information. Alternatively, you could also ask bunq support. 👍