Kong Ming should be able to predict Guan Yu's fate. Why can't he change his destiny?

It's not hard to predict. Take romance as an example. Before Zhuge Liang went to Sichuan to support Liu Bei, he gave Guan Yu the position of commander-in-chief of Jingzhou according to Liu Bei's meaning, clearly told him the strategy of refusing Cao Cao in the north and Sun Quan in the east, and repeatedly reminded him of the strategic significance of Jingzhou. It can be seen that Zhuge Liang was not at ease with Guan Yu, because Zhuge Liang knew very well how arrogant Guan Yu was, and his attitude towards Sun Wu was even worse than that of Cao Wei, but these did not mean that Zhuge Liang had the ability to change reality. Zhuge Liang is not a god. He is just a minister, and he must obey orders. Besides, there was really no better general under Liu Bei at that time, except Guan Yu, who had to use it if he didn't want to. To put it bluntly, he just resigned himself to his fate as best he could.