What should a Muslim do regarding the disagreement among scholars on fatwas, and why do they not agree on a general fatwa even though Islam is one Ummah?

You must choose the scholar you ask based on his knowledge, piety, and adherence to the Sunnah, then accept his fatwa after you are reassured by it. There is no objection to asking him for the evidence to put your heart at ease, and he should explain it to you. Do not worry about the differences among scholars in subsidiary issues, for it is a difference of diversity and mercy, indicating the breadth of the religion, while the fundamental principles and constants remain areas of consensus.