Oh, sorry. I just tried with your model... Apparently this is an error with STM32F446 on CubeMX (works fine with my STM32F405VG).If you want to enable manually, go to MX_ADC1_Init(void) function, and change "hadc1.Init.DMAContinuousRequests = DISABLE...

You need to add a DMA request for ADC on "DMA Settings" tab. After selecting a DMA stream, you'll be able to enable continuous DMA requests.