Your model has two components – a pair of electrodes and the piezoelectric substrate atop which they reside. Both the electrodes and piezoelectric substrate are modeled with PLANE223, keyoption(1)=1001. There is a new feature for PLANE223 at v11 – electroelastic coupling (useful for the simulation of electrostatically actuated MEMS devices). This is now the default behavior for PLANE223 when keyopt(1)=1001 AND when no piezoelectric properties are defined [TB,PIEZ]. For this option, the element nodal electric reaction (or “through variable” if you prefer) is positive charge (see table 223.2 in the V11 Elements Reference). The electrodes in your model were defined in this manner. If you define a piezo matrix [TB,PIEZ], this option of PLANE223 becomes piezoelectric and has negative electriccharge as its nodal electric reaction. This is the option you used to define the piezoelectric substrate in your model. The conflicting reaction variables make the results that would be obtained from this combination of element types meaningless, hence the error trap.