Hello @Ricky1,
Could you please check which value does the CLKS field have before changing the sixth bit? An CLKS 11b value is Reserved and could create issues. You could change the whole field with:
MCG->C1 = ((MCG->C1 & ~(MCG_C1_CLKS_MASK))) | (MCG_C1_CLKS(kMCG_ClkOutSrcOut);
Also, this configuration could be made in BOARD_InitBootClocks(). That part of the code usually takes care of clock settings.
Best regards, Raul.