During compilation, Simulink makes the best guess of the dimension of each signals based on its source. However, when there are algebraic loops or if-else statements involved, Simulink might struggle to find the signal dimension.
In these situations, enable 'warning' or 'error' diagnostics for the underspecified signal dimensions, as suggested in the error message. This displays additional messages to help debug the issue. To enable this diagnostics, press Ctrl+E from the Simulink model, this opens the Configuration Parameters for the model, click Diagnostics > Data Validity > Unspecified dimensions and change the diagnostics from 'none' to 'warning' or 'error'.
For the blocks with underspecified signal dimension, you can also manually specify it through Block Parameters > Signal Attributes > Port Dimensions.
