In the U.S. this is a controversial topic online because it centers around the idea of privilege. One of the progressive viewpoints around race is CRT (critical race theory). That argues that throughout history white people have created and benefitted from a system that oppresses other races and gives themselves advantages. In my opinion that's not very debatable. That being said I personally believe in the current day that social classes and education levels are what should be looked at more but race is a large part of the discussion. CRT is very popular with the very vocal left leaning Twitter folks and internet users.

Someone who is a proponent of CRT would argue that you can't be racist against the "dominant" (not the word in really lookin for) race within a state/country. An easy example of this is the lack of any kind of word that equates to a racial slur for white people. There really isn't one, because due to power dynamics throughout Americas history there hasn't been a time where white people were the ones being persecuted against and marginalized (which is where slurs and race based insults come from generally).

Where this falls apart is that even the idea of "whiteness" has changed over the last 150 years. For a long time the Irsh, Italians, Eastern Europeans (who would now just be considered white) did have a lot of persecution against them and there was significant "racial" bias against them. In reality those immigrants had much more in common with other poor people (regardless of race). But again, an Italian immigrant still usually had it better than a black person in many parts of the country (in terms of how they were viewed by most of society).

TLDR: Can't be racist against white people because they are the ones that "run" the system and hold the power in society. I don't agree with this 100% but get the merits of the argument.