Hello,
I have checked your code and you use |= when you set the clock source, but the SCS bit field is already configurated as Fast IRC (FIRC_CLK) by default.
The solution:
SCG->RCCR = SCG_RCCR_SCS(6) | SCG_RCCR_DIVCORE(0x01) | SCG_RCCR_DIVBUS(0x01) | SCG_RCCR_DIVSLOW(0x03);
Also, you did not set SPLL dividers, please, refer to RM rev 11. Table 27-1. Clock descriptions.
hello_clocks_S32K144 example located in the S32DS can be useful for you.
I hope it helps.
Best regards,
Diana