problems with Blue Pill (stm32f103c8t6) direct register manipulation
you need to use something like this:
ADC1-&gt;CR2 = ... ;

note 2 things:

you need to use ADC1, ADC2 ... to access the registers. not just ADC.
you need to use arrow (-&gt;) operator to access the registers

and you may need to include the header file, something like this :
#include &lt;stm32f030x6.h&gt;

find the proper header file for your MCU. (it's not the same as stm32f1xx, but if proper definitions are made somewhere, it may also work)