The letter says that you have been invited to file a tax return for the year 2004. The deadline already passed and the tax return has not been received yet by the tax office.
You are allowed to file the tax return within 4 weeks after the date of the reminder which is 1 September 2005. This means that the tax return must be filed roughly before 1 October. If the tax return is not filed before that date the tax office will estimate your income and you may receive a fine up to € 1,000.
A time extention will not be granted anymore.
If you already filed your tax return recently then the reminder is not relevant to you. It takes the tax office some weeks to confirm that the tax return is received in their system.
Expatax can assist you with filing your tax return.