The terminology used by ANSYS and every other Finite Element program is that a linear beam element has two nodes with a quadratic displacement function, while a quadratic beam element has three nodes and a cubic displacement function.
Two nodes can deliver quadratic displacements because each node has rotation degrees of freedom as well as translation degrees of freedom.
The benefit of a quadratic beam element is that it can represent curved geometry, while a linear beam element can only represent geometry that is initially straight.
You seem to be looking for a two node element that has a linear displacement function. That is called a truss element, which NASTRAN calls a ROD and ANSYS calls a Link Element, or LINK180. That has only translation degrees of freedom and no rotation degrees of freedom, so the displacement function is linear. In the Details of the Line body under Geometry, you can change a Beam to a Link.
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