When connected to the debugger, MCU enters an emulated STOP mode. Try disconnecting and power-cycling the MCU. It is described in this other community post by my colleague: Solved: Cannot enter VLPS mode - NXP Community.
You can refer to the power_mode_switch_s32k144 example for the mode switch routine. There is also AN5425: Power Management for S32K1xx; chapter 5.5.2. shows the VLPS entry routine: