URGENT: Cannot change systick frequency STM32F746

Good day,

I am new to this forum.  Please help. I want to change the systick frequency, but when I call from main.c the function "SystemClock_Config" I get " Stopped by Vector Catch" in a loop.  When I step through the code, it seems that the problem is when the PLL is started up again after the settings have been changed. i.e. when the function "__HAL_RCC_PLL_ENABLE();" is called after the call to "WRITE_REG(...)"'

Error: main.c -> SystemClock_Config() -> if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)  -> __HAL_RCC_PLL_ENABLE()

The SystemClock_Config function was generated by the MXCube program.

What could be the problem?  

Any help will be appreciated!

Thanks