The error show this "Some higher propagating modes at port 1 are not considered time domain calculation. This possibly leads to an inaccurate energy balance. Consider to increase the mode or decrease upper frequency limit."
Some options: First, increase the number of modes for the Waveguide and look at which frequency this mode is propagating. Maybe you can reduce your frequency span. Second, change the dimension of the waveguide!
but I do not know : how I can increase the number of modes for the waveguide port in CST ?
When I run the project in transient solver ,I choose all modes for solving !