For Conan 2d20 this is actually the case, you always make a Skill test, never an Attribute test.
For John Carter of Mars 2d20 you never make a Skill test, there are no Skills there, only Attributes, and you always roll against a Target Number of the sum of two Attributes.
Those are - depending on which 2d20-based game you are looking at - some basic principles that always apply.
As I said, if you need to make a skill test and you don’t have the Skill, then you roll your Attribute + 0 (no ranks in Expertise for this Skill), so your Target Number is identical to your Attribute value, BUT (big “but” here) your Complication range is increased, too!
So it might look like an Attribute test, if your Target Number is identical to the Attribute value, but it still is a Skill test, and you still have no Expertise, so you still have an increased Complication range.
If one were to allow a pure Attribute test, this would clash with penalizing the characters who don’t have a Skill and who get an increased Complication range, as it is nowhere defined how to make an Attribute test in Conan 2d20, so the Complication range increase would not apply, as this is linked to the lack of Skill Expertise.
I stand by NEVER make an Attribute test in Conan 2d20, as this is not part of the Conan 2d20 rules. (Other 2d20-based games may differ.)
You could, of course, house rule this, but then some things like replacement skills etc. would no longer apply, Complication range increase would be applied differently or not at all, so mechanics will start to chafe and cause problems. Your game, your rules. The Conan 2d20 core rules as written never use Attribute tests, always Skill tests.
Situation: the character is required to participate in a formal dance of the noble debuttantes at the court of Aquilonia, hence the Society Skill is used.
This is not to impress someone, but to correctly perform the required steps to show you are a character of learning, of control and virtue. So this is not the usual challenge of Personality + Society, but more the Coordination + Society combination (like in the stage fighting aspects of Coordination + Parry, see the respective Talents there, though this is not a “choreographed fight”, it is an actual choreography for a dance in a stressful societal environment at the royal court).
The character with Coordination 11 and Society Exp 2, Foc 1 would roll the dice pool against a Target Number of: Attribute + Skill Expertise Rank = Coordination + Society = 11 + 2 = 13, having Focus 1 this is a roll of TN 13 (1).