Hi,
as RM states, "The STO interrupt flag in the Interrupt Flag and Status Register 2 (eSCI_IFSR2) will be
set, if an LIN RX frame was not fully received in the amount of time specified in the timeout value field TO in the LIN Transmit Register (eSCI_LTR)."
So set TO field of 3rd and 4th word written to LTR accordingly (see LTR register description), for recommended TO value refer to chapter 49.5.6.5.5 Slave-Not-Responding-Error Detection of the RM. For tests you can set TO to max value, i think.
Before sending another frame STO should be cleared.
BR, Petr