Hi Henrique,
In the following section:
MAPDL Help -> Material Reference -> 4.8.3.1.2 Experimental Data Complex Modulus
Towards the bottom:
Two elastic constants are required to define the model. If only one set of experimental data is defined, the poisson's ratio is used as the second elastic constant.
You can add the following to your command snippet.
  MP, PRXY, MATID, 0.3  ! 0.3 being the possions ratio for your material
Alternatively you can use the TB command, or add bulk modulus, tensile modulus, or possions ratio as experimental data.
If this doesn't remove the error, can you please add screenshots of your material definition in engineering data and mechanical showing the command snippet implementation.
You are navigating away from the AIS Discovery experience