"the main task of" is correct and can be used in written English.
For example, "The main task of our team is to finish the project on time."
Fact-checking was the main task of a researcher.
The main task of the king is to offer protection.
The main task of the Council is to regulate and approve European laws.
The main task of his FAF circle is to keep him grounded.
The main task of economics has been to understand this astonishing process.
The main task of the correspondent was to produce articles showing the heroism of the Soviet soldiers.
In my view, the main task of architecture today is to mediate between our inner and outer worlds.
She feared the development of a military 'substructure' that would divert effort and resources from the main task of strengthening the transatlantic Western Alliance.
But the main task of swaying the public falls to Tomas Klvana, who previously worked for British American Tobacco and is known here as Mr. Radar.
To make matters worse, during a news conference on Tuesday Mr. Ma suggested that the main task of Taiwan's army should be prevention and rescue.
"The main task of any commander is not just to prepare an order, but to ensure that it is carried out," Mr. Kulikov said caustically.
