Islam is against taxation systems, as it says in https://islamqa.info/en/130920, then what are the sources of income of the government of an Islamic country?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

First of all, it is not the specialization of the Fatwa Center to evaluate or analyze the fatwas of other websites or authorities.

As regards the ruling on imposing taxes, then the basic principle is that - without a doubt - imposing taxes is consuming the money of the Muslims unjustly, as their money becomes permissible only if they give it with their own proper consent, unless the general resources of the country do not meet the needs of the nation and the people's interests and the state imposes these taxes in order to fulfill these needs, and really fulfill them. In this case, it is permissible for the ruler to impose such taxes, but only to the extent of meeting the need, without any injustice or exaggeration. For more benefit, please refer to fatwas 83863 and 85356.  

As regards the financial resources of the Islamic state, they are many and diverse, both in the past and present; among them is: the new natural resources that are discovered in the territory of the country, such as oil and gas and types of minerals, such as gold, copper, iron, etc. Among the resources as well is zakah of all kinds, and the fifth of the produce of the land from minerals, gold, silver, iron, etc.

Also, among the resources is the produce of the land of the treasury and its properties, and the profit that results from trading with it.

For more details, please refer to these resources in the books of Fiqh, such as the Kuwaiti Fiqh Encyclopedia, under the title: The Resources of the Treasury; volume number 8.

Allah knows best.