Donald Trump wanted to change the rules. Instead of counting every person living in the United States (as has always been done in the past), Trump issued an order that only citizens should be counted to determine each state's representation in Congress.
This would have substantially changed how some states were represented -- those state with a large undocumented immigrant population.
Trump's order was appealed to the U.S. Supreme Court. The court refused to rule on the matter, saying the change had not happened, and might not happen (depending on when the census numbers were reported to the president).
It looks like the court might have foreseen the case was moot. Now the Census Bureau is saying they cannot report the numbers by the end of the year (2020). Instead, they will probably report those numbers by the end of January 2021. That means the numbers would be reported to President Joe Biden -- not Trump.
Joe Biden will then be the one to report the census numbers, and he will report all people -- not just citizens.
It will be the final failure (among many) for the Trump administration.