The sample length should be exactly equal to the distance between the support.

You didn't show a coordinate system, so I will assume that the sample length is along the X axis, the Y axis is up and the sample width is along the Z axis.

On the left end bottom edge of the sample, use a Displacement and set X, Y, Z to 0.

On the right end bottom edge of the sample, set Y and Z displacements to 0.

Now you have sufficient constraints and the system will solve if you apply a force to the row of nodes in the center on the sample on the top face.