You can no longer log in with a user name and password issued by the Dutch Tax and Customs Administration.
To file a tax return as a non-resident tax-payer, you can only log in with your DigiD or an EU-approved login key from your country of residence.
Do you not yet have a DigiD or an EU-approved login key?
Apply for one. Read more on How do I file an income tax return as a non-resident taxpayer?