u/CarbonInTheWind and this comment kinda sum it up nicely.
Basically, when people refer to racism there are two kinds. The one most people actually mean is prejudice. Anyone can be prejudiced against, even white people. Everyone can suffer negative experiences caused by someone with negative views on your skin color.
The second type (and the one the people in your post mean) is systemic/institutional racism. White people aren’t affected by this. A white person doesn’t get denied a job because of their skin color, a white person doesn’t make less money because of their skin color, a white person isn’t subject to “random spot checks” due to their skin color, a white person doesn’t have a higher likelihood to be falsely accused and imprisoned due to their skin color (etc etc etc).
This is what people mean when they say white people cant suffer from racism. The system is designed for them to be at the top, being the majority.