Guilin Sun
Ansys Employee

There are many reasons however please check if the following settings are correct:

1: the monitor is inside the simulation region. If it is 2D simulation please check the z position of the monitor is the same as FDTD.

2: If you are using analysis group, please open it by right clicking it, and run the analysis script (make sure it is in the script window). If there is any error in the script it will show.

Please note that your description lacks some information. If you provide screenshots, and error message it would be great helpful for us to check and you can get a quick resolution.