I have replied to you via email but will copy the response here for others dealing with this issue.
This occurs when the waveguide was not properly set or the waveguide that was selected was deleted after selection. Please review section 2.2.6 in the tutorial document and verify that you followed these steps. If you are still having issues a completed version of this design is located at ” OptiFDTD 14 Samples\2D_samples\photonic_components\TE_coupler.fdt” within your documents folder provided that you installed the samples when you installed the product.