SOLVED! (with the excellent help of Mathworks Technical Support)
1. Pins GPIO42 and GPIO43 must support SCI_A. For this board, we expect that pins GPIO42 (TX) and GPIO43 (RX) have been configured for SCI_A communication (TX and RX, respectively). Can you please confirm that this is the case?
2. The oscillator frequency must match the LaunchPad's external oscillator frequency. While the F28379D has an internal crystal that oscillates at 20MHz, the LaunchPad also has an external oscillator that runs at 10MHz. Simulink's default value is 20MHz. This setting is appropriate when working with the F28379D alone. However, this frequency value can cause problems when using external mode on a LaunchPad. Please go to Hardware Implementation -> Device details -> Clocking and change the value of "Oscillator clock (OSCCLK) frequency in MHz" from 20 to 10.